Upcoming Events (Archive)

Many events are currently organized online. Information on how to access these events can be found by clicking “more” below the respective entry.


go to current events

Wednesday, 13.10.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Tejas Iyer, WIAS Berlin:
Degrees of fixed vertices and power law degree distributions in preferential attachment trees with neighbourhood influence (online talk)
more ... Location
Online Event

Abstract
Motivated by the structure of complex networks such as the internet, we consider a growing model of preferential attachment trees with neighbourhood influence, where vertices arrive one at a time, are equipped with independent weights, and connect to existing vertices with probability proportional to their fitness function: a function of their own weight and the weights of their neighbours. In this model we prove almost sure limiting statements for the proportion of vertices with a given degree having weight belonging to a given measurable set, and the proportion of edges in the tree with endpoint belonging to a measurable set. We show that under certain conditions, the latter quantity demonstrates a condensation phenomenon, in which a positive proportion of edges in the network accumulate among those of weight that confers maximal reinforcement of fitness. Finally, we derive almost sure limiting statements for the log of the degree of a fixed vertex in this model, and prove that in this model the degree distribution behaves like a power law - a ubiquitous feature of many real-world complex networks.

Further Informations
Seminar Interacting Random Systems (Online Event)

Host
WIAS Berlin
Wednesday, 29.09.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Michiel Renger, WIAS Berlin:
Open problem: large deviations of transport maps (online talk)
more ... Location
Online Event

Abstract
Motivated by nonequilibrium thermodynamics, we are interested in large deviations for transport maps as the number of particles n goes to infinity. We first focus on the simplest setting of independent Markovian particles on a finite state space, which already poses mathematical challenges. Let n Γ _xy(s,t) be the number of particles that were at position x at time s and at position y at time t. Common large-deviation techniques for random trajectories exploit the Markov property (in one time variable). However, Γ has two time dimensions; can we reformulate Γ as a Markov process?

Further Informations
Seminar Interacting Random Systems (Online Event)

Host
WIAS Berlin
Wednesday, 15.09.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Alexander Zass, WIAS Berlin:
Gibbs point processes on path space: existence, cluster expansion and uniqueness (online talk)
more ... Location
Online Event

Abstract
In this talk we present a class of infinite-dimensional diffusions under Gibbsian interactions, viewed in the context of marked point configurations: the starting points belong to R^d, and the marks are the paths of Langevin diffusions. We use the entropy method to prove existence of an infinite-volume Gibbs point process and use cluster expansion tools to provide an explicit activity domain in which uniqueness holds.

Further Informations
Seminar Interacting Random Systems (Online Event)

Host
WIAS Berlin
August 23 – 27, 2021 (Schloss Dagstuhl)
Workshop/Konferenz: Leibniz MMS Summer School 2021 “Mathematical Methods for Machine Learning”
more ... Location
Leibniz-Zentrum für Informatik, Schloss Dagstuhl

Abstract
Fundamental and advanced aspects of ML methods are presented with the aim to enable the participants to apply such methods to their specific research problems. The topics that will be discussed include kernel methods and Gaussian processes, stochastic and robust optimization, deep Neural Networks. In addition to daily lectures, the participants will get the opportunity to become familiar with the presented methods in accompanying practical lab sessions.

Host
WIAS Berlin
Tuesday, 17.08.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Jia-Jie Zhu, WIAS Berlin/MPI-IS Tübingen:
Kernel methods for distributionally robust optimization and machine learning (online talk)
more ... Location
Online Event

Abstract
Learning under distribution shift brings new challenges that cannot be addressed by canonical statistical learning theory. In this talk, I will present the use of reproducing kernel Hilbert spaces (RKHS) as the dual spaces for enforcing distributional robustness in stochastic optimization and machine learning. I then prove a generalized strong duality for distributionally robust optimization (DRO) using the integral probability metrics (IPM), of which the type-1 Wasserstein metric is a special instance. Our analysis highlights the roles that smooth function majorants, such as RKHS functions and Moreau envelope, play in enforcing distributional robustness in optimization. Finally, I will introduce a new class of kernel semi-infinite programs that originated from the DRO reformulation.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 10.08.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Darina Dvinskikh, WIAS Berlin:
Decentralized algorithms for Wasserstein barycenters (online talk)
more ... Location
Online Event

Abstract
We consider the Wasserstein barycenter problem of discrete probability measures from computational and statistical sides in two scenarios: (i) the measures are given and we need to compute their Wasserstein barycenter, and (ii) the measures are generated from a probability distribution and we need to calculate the population barycenter of the distribution defined by the notion of Fréchet mean. The statistical focus is estimating the sample size of measures necessary to calculate an approximation for Fréchet mean (barycenter) of a probability distribution with a given precision. For empirical risk minimization approaches, the question of the regularization is also studied together with proposing a new regularization which contributes to the better complexity bounds in comparison with quadratic regularization. The computational focus is developing algorithms for calculating Wasserstein barycenters: both primal and dual algorithms which can be executed in a decentralized manner.

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Thursday, 15.07.2021, 14:00 (Online Event)
Seminar Numerische Mathematik
Prof. Marc-Andre Keip, Universität Stuttgart:
Multiscale analysis of instability-induced pattern transformations of composite active solids
more ... Location
Online Event

Abstract
We discuss instability phenomena of active solids with periodic microstructures. Depending on the material, instabilities can be triggered by different means. In a first part of the talk, we analyze instability-induced pattern transformations of electroactive polymer composites and magnetorheological elastomers. In a second part, we widen the view towards the transient analysis of diffusion-driven instabilities of hydrogels, where we study pattern transformations of composite hydrogel thin films at different length scales.
Acknowledgements: Elten Polukhov, Siddharth Sriram
References
[1] G. Geymonat, S. Müller and N. Triantafyllidis. Homogenization of nonlinearly elastic materials, microscopic bifurcation and macroscopic loss of rank-one convexity. Archive for Rational Mechanics and Analysis 122:231-290 (1993)
[2] K. Bertoldi, M. Boyce, S. Deschanel, S. Prange and T. Mullin. Mechanics of deformation-triggered pattern transformations and superelastic behavior in periodic elastomeric structures. Journal of the Mechanics and Physics of Solids 56:2642-2668 (2008)
[3] C. Tipton, E. Han and T. Mullin. Magneto-elastic buckling of a soft cellular solid. Soft Matter 8:6880-6883 (2012)
[4] Wu, G., Xia, Y., Yang, S. Buckling, symmetry breaking, and cavitation in periodically micro-structured hydrogel membranes. Soft Matter 10:1392-1399 (2014)
[5] Polukhov, E., Vallicotti, D. and Keip, M.-A. Computational stability analysis of periodic electroactive polymer composites across scales. Computer Methods in Applied Mechanics and Engineering 337:165-197 (2018)
[6] Polukhov, E. and Keip, M.-A. Multiscale stability analysis of periodic magnetorheological elastomers. Mechanics of Materials, 159:103699 (2021)
[7] Sriram, S., Polukhov, E. and Keip, M.-A. Transient stability analysis of composite hydrogel structures based on a minimization-type variational formulation. International Journal of Solids and Structures, 111080 (2021)

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 14.07.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Xin Liu, WIAS Berlin:
Well-posedness of Hibler's dynamical sea-ice model (online talk)
more ... Location
Online Event

Abstract
We establish the local-in-time well-posedness of solutions to an approximating system constrcuted by mildly regularizing the dynamical sea ice model of W.D. Hibler in 1979, which is widely used in the prediction of evolution of Artic sea ice. Our choice of regularization has been carefully designed, prompted by physical considerations, to retain the original coupled hyperbolic-parabolic character of Hibler's model. The well-posedness theory of such a system provides a first-step groundwork in both numerical study and future analytical study. This is joint work with Marita Thomas and Edriss Titi.

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 13.07.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Prof. Dr. Messoud Efendiyev, Helmholtz Zentrum München:
Mathematical modeling of biofilms and their long-time dynamics (online talk)
more ... Location
Online Event

Abstract
In this talk we deal with mathematical modelling of biofilms, that show biofilm performance is non-uniform. Moreover, our model leads to a new class of degenerate PDEs. Effect of degeneracy to large time behavior of solutions will also be considered.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://us02web.zoom.us/j/84203723799?pwd=bmNVcVBaazRzSlI3TjAxV1U1S2xFQT09

Host
Freie Universität Berlin
WIAS Berlin
July 8, 2021 (Online Event)
Workshop/Konferenz: Minisymposium ”Perspectives of Gradient-Robustness”
more ... Location
Online Event

Host
WIAS Berlin
Wednesday, 07.07.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Klaas Hendrik Poelstra, Technische Universität Dortmund:
Dimension reduction for elastoplastic rods and homogenization of elastoplastic lattices (online talk)
more ... Location
Online Event

Abstract
We derive effective equations for periodic lattices of linearly elastoplastic rods in the limit of both infinitesimal periodicity and infinitesimal relative width of the rods. For this derivation we use the method of evolutionary Gamma-convergence for quadratic rate-independent systems. As a first step towards that goal we derive effective equations for a single rod. After introducing appropriate scalings, the main difficulty lies in the proof of Gamma-convergence for the stored energy. For the study of periodic lattices we then introduce the notion of periodic graph frameworks, discuss infinitesimal rigidity properties of such frameworks and define a simple notion of two-scale convergence. The stored energy of a lattice of rods ist just the sum of the energies of the individual rods, coupled by boundary conditions at the nodes. For this energy we again prove Gamma-convergence. In the presence of volume loads we observe qualitatively different behaviour depending on the relative rate of convergence of the periodicity parameter and the thickness parameter.

Further Informations
Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 07.07.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Daoyi Wang, Leiden University:
The Parabolic Anderson Model on a Galton-Watson tree with unbounded degrees (online talk)
more ... Location
Online Event

Abstract
A detailed analysis was given by den Hollander, König and dos Santos (2020), of the large-time asymptotics of the total mass of the solution to the parabolic Anderson model on a supercritical Galton-Watson random tree, with an i.i.d. random potential whose marginal distribution is double-exponential. Under the assumption that the degree distribution has bounded support, two terms in the asymptotic expansion were identified under the quenched law, i.e., conditional on the realisation of the random tree and the random potential. We extend the analysis to degree distributions with unbounded support. We identify the weakest condition on the tail of the degree distribution under which the arguments can be pushed through. This is joint work with my supervisor Frank den Hollander.

Further Informations
Seminar Interacting Random Systems (Online Event)

Host
WIAS Berlin
Wednesday, 07.07.2021, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Dr. Victor Panaretos, École Polytechnique Fédérale de Lausanne, Schweiz:
Testing for the rank of a covariance operator (online talk)
more ... Location
Online Event

Abstract
How can we discern whether the covariance operator of a stochastic process is of reduced rank, and if so, what its precise rank is? And how can we do so at a given level of con dence? This question is central to a great deal of methods for functional data, which require low-dimensional representations whether by functional PCA or other methods. The difficulty is that the determination is to be made on the basis of i.i.d. replications of the process observed discretely and with measurement error contamination. This adds a ridge to the empirical covariance, obfuscating the underlying dimension. We describe a matrix-completion inspired test statistic that circumvents this issue by measuring the best possible least square fit of the empirical covariance's off -diagonal elements, optimised over covariances of given nite rank. For a fixed grid of sufficiently large size, we determine the statistic's asymptotic null distribution as the number of replications grows. We then use it to construct a bootstrap implementation of a stepwise testing procedure controlling the family-wise error rate corresponding to the collection of hypotheses formalising the question at hand. Under minimal regularity assumptions we prove that the procedure is consistent and that its bootstrap implementation is valid. The procedure circumvents smoothing and associated smoothing parameters, is indi fferent to measurement error heteroskedasticity, and does not assume a low-noise regime. (Based on joint work with Anirvan Chakraborty.)

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 06.07.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Prof. Dr. Carsten Conradi, Hochschule für Technik und Wirtschaft Berlin:
Monomial parameterizations in the analysis of biochemical reaction networks (online talk)
more ... Location
Online Event

Abstract
The dynamics of biochemical reaction networks can be described by ODEs with polynomial right hand side. In this presentation networks are considered where the steady state variety can be parameterized by monomials. I present two applications of these monomial parameterizations in the analysis of reaction networks: (i) deciding multistationarity and (ii) establishing Hopf bifurcations. Here multistationarity refers to the existence of at least two positive solutions to the polynomial steady state equations. And if a monomial parameterization exists, then this question is equivalent to the feasibility of at least one linear inequality system (out of many). The results presented here can be used to determine parameter values where multistationarity or Hopf bifurcations occur.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://us02web.zoom.us/j/84203723799?pwd=bmNVcVBaazRzSlI3TjAxV1U1S2xFQT09

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 06.07.2021, 10:15 (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Luisa Plato, WIAS Berlin:
Biological pest control -- Analysis and numerics for a spatio-temporal predator-prey system
more ... Location
Online Event

Further Informations
You can follow this talk by zoom: https://zoom.us/j/98493865627.

Host
WIAS Berlin
Monday, 05.07.2021, 15:00 (Online Event)
Berlin Oberseminar: Optimization, Control and Inverse Problems
Assoc. Prof. Patrick E. Farrell, University of Oxford, UK:
Computing disconnected bifurcation diagrams of partial differential equations
more ... Location
Online Event

Abstract
Computing the distinct solutions u of an equation f(u, lambda)=0 as a parameter lambda in IR is varied is a central task in applied mathematics and engineering. The solutions are captured in a bifurcation diagram, plotting (some functional of) u as a function of lambda. In this talk I will present a new algorithm, deflated continuation, for this task. Deflated continuation has three advantages. First, it is capable of computing disconnected bifurcation diagrams; previous algorithms only aimed to compute that part of the bifurcation diagram continuously connected to the initial data. Second, its implementation is very simple: it only requires a minor modification to an existing Newton-based solver. Third, it can scale to very large discretisations if a good preconditioner is available; no auxiliary problems must be solved. We will present applications to hyperelastic structures, liquid crystals, and Bose-Einstein condensates, among others.

Further Informations
Berlin Oberseminar: Optimization, Control and Inverse Problems

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 30.06.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Stefanie Schindler, WIAS Berlin:
Self-similar diffusive equilibration for a coupled reaction-diffusion system with mass-action kinetics (online talk)
more ... Location
Online Event

Abstract
We study a coupled system of two nonlinear reaction-diffusion equations on the unbounded domain ℝ, where we consider reaction kinetics of the reaction nX1 ⇌ mX2 according to the mass-action law. We show the global existence and uniqueness of classical solutions and prove the existence of invariant rectangles, that is, bounded regions ∑ ⊂ ℝ2 in which the solutions stay for all times, provided that the initial data lies entirely in ∑. Further, by assuming that the reaction is equilibrated and that the asymptotic boundary conditions at infinity are given by constant steady-state solutions, the diffusive mixing is studied. Here the theory of self-similar solutions in the sense of Barenblatt is applicable. Combined with the theory of monotone operators, this enables us to prove that there exists for each choice of asymptotic boundary conditions a similarity profile. This research is joint work with Alexander Mielke.

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 29.06.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Prof. Sebastian Wieczorek, University College Cork:
Rate-induced tipping points (online talk)
more ... Location
Online Event

Abstract
Many systems are subject to external disturbances or changing external conditions. For a system near a stable state (an attractor) we might expect that, as external conditions change with time, the stable state will change too. In many cases the system may adapt to changing external conditions and track the moving stable state. However, tracking may not always be possible owing to nonlinearities and feedbacks in the system. So far, the focus has been on critical levels of external conditions (dangerous autonomous bifurcation points) where the stable state turns unstable or disappears, causing the system to suddenly move to a different and often undesired state. We describe this phenomenon as bifurcation-induced tipping or B-tipping. However, critical levels are not the only critical factor for tipping. Some systems can be particularly sensitive to how fast the external conditions change and have critical rates: they suddenly and unexpectedly move to a different state if the external input changes too fast. This happens even though the moving stable state never loses stability in the classical autonomous sense! We describe this phenomenon as rate-induced tipping or R-tipping. Being a genuine non-autonomous bifurcation, R-tipping is not captured by the classical bifurcation theory and requires an alternative framework. In the first part of the talk, we demonstrate R-tipping in a simple ecosystem model where environmental changes are represented by time-varying parameters. We then introduce the concept of basin instability and show how to complement the classical bifurcation diagram with information on nonautonomous R-tipping that cannot be captured by the classical bifurcation analysis. In the second part of the talk, we develop a general mathematical framework for R-tipping with decaying inputs based on the concepts of thresholds, edge states and special compactification of the nonautonomous system. This allows us to transform the R-tipping problem into a connecting heteroclinic orbit problem in the compactified system, which greatly simplifies the analysis. We explain the key concept of threshold instability and give rigorous testable criteria for R-tipping to occur in arbitrary dimension. In the third part of the talk, we discuss the so-called “compost-bomb instability", which is an example of R-tipping without an obvious threshold. We use geometric singular perturbation theory and desingularisation to reveal non-obvious R-tipping thresholds and edge states.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 29.06.2021, 10:15 (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Ingo Bremer, WIAS Berlin:
Parameterbestimmung zur Anpassung des SEIR-Models an Covid-19 Zeitreihen
more ... Location
Online Event

Further Informations
You can follow this talk by zoom: https://zoom.us/j/98493865627.

Host
WIAS Berlin
Thursday, 24.06.2021, 13:00 (Online Event)
Seminar Numerische Mathematik
Felipe Galarce Marín, WIAS Berlin:
Inverse problems in haemodynamics. Fast estimation of blood flows on non-parametric domains from medical data
more ... Location
Online Event

Abstract
This seminar presents a work at the interface between applied mathematics and biomedical engineering. The work's main subject is the estimation of blood flows and quantities of medical interest in diagnosing certain diseases concerning the cardiovascular system. We propose a complete pipeline, providing the theoretical foundations for state estimation from medical data using reduced-order models, and addressing inter-patient variability. Extensive numerical tests are shown in realistic 3D scenarios that verify the potential impact of the work in the medical comunnity.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 23.06.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Martin Brokate, Technische Universität München:
A variational inequality for the derivative of the play operator (online talk)
more ... Location
Online Event

Further Informations
Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 23.06.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Tejas Iyer, WIAS Berlin:
Degree and Edge Distributions in Inhomogeneous Random Recursive Trees (online talk)
more ... Location
Online Event

Abstract
We introduce general models of inhomogeneous random recursive trees, where either one or several nodes arrive at a time, and are equipped with i.i.d random weights. At each time-step, an existing vertex is chosen with probability proportional to its fitness function, and the newly arriving node(s) connect to it. This may be a function of its weight, and possibly the weights of its neighbours. We study two main quantities: the empirical measures associated with the number of vertices with a given degree and weight and the empirical measure corresponding proportion of edges in the structure with endpoint having a given weight. We show that, under certain technical conditions, the limit of both quantities exists when normalised by the size of the network, as the number of nodes tends to infinity. However, when the trees take certain forms, we show that interesting, non-trivial behaviour can emerge when these conditions fail: in particular, the trees may exhibit emphcondensation where a positive proportion of edges accumulate around vertices with weight that maximises the reinforcement of their fitness, or, more drastically, may have a emphdegenerate limiting degree distribution where the entire proportion of edges accumulate around these vertices. This non-trivial behaviour may be of interest when considering the trees as toy models for the evolution of complex networks.

Further Informations
Seminar Interacting Random Systems (Online Event)

Host
WIAS Berlin
Tuesday, 22.06.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Bhumika Thakur, Jacobs University, Bremen:
Data driven identification of nonlinear dynamics using sparse regression with applications in plasma physics
more ... Location
Online Event

Abstract
Data driven techniques are increasingly finding applications in physical sciences and plasma physics is no exception. Many plasma processes are highly complex and nonlinear and often the exact form of the equations governing their dynamics is not known. If we can construct these equations from the experimental data, then we can further our understanding of these processes and use techniques such as model reduction to isolate dominant physical mechanisms. A large number of regression techniques are available for identification of system dynamics from data, with varying degrees of generality and complexity. Sparse identification of nonlinear dynamics (SINDy) algorithm is one such technique that can be used to find parsimonious models. I will talk about this algorithm and discuss some examples where it is being applied in plasma physics with a focus on our ongoing attempt at finding the model equations for anode glow oscillations observed in a glow discharge plasma device.

Host
Freie Universität Berlin
WIAS Berlin
Wednesday, 16.06.2021, 12:30 (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Irène Gijbels, KU Leuven, Netherlands:
Extremiles and extremile regression (online talk)
more ... Location
Online Event

Abstract
Quantiles and expectiles of a distribution are found to be useful descriptors of its tail in the same way as the median and mean are related to its central behavior. In this talk we discuss an alternative class to expectiles, called extremiles. The new class is motivated via several angles, which reveals its speci c merits and strengths. Extremiles suggest better capability of tting both location and spread in data points and provide an appropriate theory that better displays the interesting features of long-tailed distributions. We brie y discuss estimation of extremiles. A large part of the talk will be on regression extremiles, which thus de ne a least squares analogue of regression quantiles.We discuss estimation of conditional extremiles, in which we rely on local linear (least squares) checkfunction minimization. An asymptotic normality result for the estimators is established. Attention also goes to extending extremile regression far into the tails of heavy-tailed distributions. For this purpose extrapolated estimators are constructed and their asymptotic theory is developed. Applications to real data illustrate how extremiles and related tools can be used in practice.)

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
June 16 – 18, 2021 (Online Event)
Workshop/Konferenz: Nonlinear Dynamics in Semiconductor Lasers
more ... Location
Online Event

Host
WIAS Berlin
Tuesday, 15.06.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Philipp Lorenz-Spreen, MPI for Human Development, Berlin:
Modeling radicalization dynamics and polarization in temporal networks
more ... Location
Online Event

Abstract
Echo chambers and opinion polarization have been recently quantified in several sociopolitical contexts, across different social media, raising concerns for the potential impact on the spread of misinformation and the openness of debates. Despite increasing efforts, the dynamics leading to the emergence of these phenomena remain unclear. Here, we propose a model that introduces the phenomenon of radicalization, as a reinforcing mechanism driving the evolution to extreme opinions from moderate initial conditions. Empirically inspired by the dynamics of social interaction, we consider agents characterized by heterogeneous activities and homophily. We analytically characterize the transition from a global consensus to an emerging radicalization that depends on parameters, which can be interpreted as the controversialness of a topic and the strength of social influence people exert on each other. Finally, we offer a definition of echo-chambers via our model and contrast the model's behavior against empirical data of polarized debates on Twitter, qualitatively reproducing the observed relation between users' engagement and opinions, as well as opinion segregation based on the interaction network. Our findings shed light on the dynamics that may lie at the core of the emergence of echo chambers and polarization in social media.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 15.06.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Lennard Henze, Humboldt-Universität zu Berlin:
Classification of quantum dot images using machine learning (online talk)
more ... Location
Online Event

Abstract
A possibility of classifying quantum dot images obtained out of a simulation using a machine learning algorithm and image augmentation techniques is explored. The results on the similar MNIST data set suggest that a classification using machine learning techniques is indeed possible. Furthermore, a theoretical result concerning the extrapolation properties of artificial neural networks will be presented.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 15.06.2021, 10:15 (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Maximilian Reiter, TU Berlin:
Numerical approximation of the Ericksen--Leslie equations (online talk)
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Host
WIAS Berlin
Monday, 14.06.2021, 15:00 (Online Event)
Berlin Oberseminar: Optimization, Control and Inverse Problems
Dr. Ozan Öktem, KTH Royal Institute of Technology, Stockholm:
Data driven large-scale convex optimisation
more ... Location
Online Event

Abstract
This joint work with Jevgenjia Rudzusika (KTH), Sebastian Banert (Lund University) and Jonas Adler (DeepMind) introduces a framework for using deep-learning to accelerate optimisation solvers with convergence guarantees. The approach builds on ideas from the analysis of accelerated forward-backward schemes, like FISTA. Instead of the classical approach of proving convergence for a choice of parameters, such as a step-size, we show convergence whenever the update is chosen in a specific set. Rather than picking a point in this set through a handcrafted method, we train a deep neural network to pick the best update. The method is applicable to several smooth and non-smooth convex optimisation problems and it outperforms established accelerated solvers.

Host
WIAS Berlin
Wednesday, 09.06.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Julian Fischer, Institute of Science and Technology Austria (IST Austria):
Uniqueness and stability properties of multiphase mean curvature flow: An approach based on the variational (gradient flow) structure of the problem (online talk)
more ... Location
Online Event

Abstract
For many evolution problems for interfaces - like for instance multiphase mean curvature flow or the Mullins--Sekerka equation - appropriate weak solutions are known to exist globally in time, but the uniqueness of such weak solutions is either unknown or even known to fail. At the same time, due to geometric singularities strong solution concepts are in general limited to local in time existence results. In the absence of a comparison principle, the relation between weak and strong solutions for interface evolution problems has remained a mostly open question. We establish a weak-strong uniqueness principle for planar multiphase mean curvature flow: We prove that for mean curvature flow of planar networks,there is only a single BV solution prior to the first topology change. Our approach relies on the variational structure of mean curvature flow, being the gradient flow of the interface energy functional. For many minimization problems for interface energy functionals, the method of calibrations has made it possible to deduce the uniqueness of minimizers. Our approach to weak-strong uniqueness for mean curvature flow relies on a novel gradient-flow analogue of this concept of calibrations, basically allowing us to show that the route of steepest descent in the energy landscape is unique and stable with respect to perturbations. In the last part of the talk, we will discuss possible future applications of our concept, including the quantitative convergence of phase-field approximations for mean curvature flow.

Further Informations
Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 09.06.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Wen Sun, TU Berlin:
Pathwise large deviation for the pure jump k-nary interacting particle systems (online talk)
more ... Location
Online Event

Abstract
A pathwise large deviation result is proved for the pure jump models of k-nary interacting particle system introduced by Kolokoltsov that generalize classical Boltzmann's collision model, Smoluchovski's coagulation model and many others. The upper bound is obtained by following the standard methods of using a process "perturbed" by a regular function. To show the lower bound, we propose a family of orthogonal martingale measures and prove a coupling for the general perturbations. The rate function is studied based on the idea of Léonard with a simplification by considering the conjugation of integral functionals on a subspace of L^infty.. General "gelling" solutions in the domain of the rate function are also discussed.

Further Informations
Seminar Interacting Random Systems (Online Event)

Host
WIAS Berlin
Tuesday, 08.06.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Chunming Zheng, MPI for Physics of Complex Systems, Dresden:
Transition to synchrony in the three-dimensional Noisy Kuramoto model
more ... Location
Online Event

Abstract
We investigate the transition from incoherence to global collective motion in a three-dimensional swarming model of agents with helical trajectories, subject to noise and global coupling. Without noise this model was recently proposed as a generalization of the Kuramoto model and it was found, that alignment of the velocities occurs for arbitrary small attractive coupling. Adding noise to the system resolves this singular limit.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 01.06.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Dr. Georgi Medvedev, Drexel University:
Unfolding chimeras: Where Turing meets Penrose (online talk)
more ... Location
Online Event

Abstract
The coexistence of coherence and incoherence is arguably the most interesting effect in the theory of synchronization and possibly in nonlinear science in general discovered in the past two decades. Despite intense research chimera states still present many challenging questions to the nonlinear science community. There is no consensus on how to define chimera states. Further, the theory is only available for chimera states lying in the Ott--Antonsen manifold, which is an elegant but a very special case. In this work, we suggest a new way for studying chimera states based on the combination of the linear stability analysis of mixing and a beautiful method of Penrose for Vlasov equation in plasma physics. This approach yields a new qualitative description of chimera states and provides very accurate quantitative estimates. Our results are universal in the sense that the structure and bifurcations of chimera states are explained in terms of the qualitative properties of the distribution of intrinsic frequencies and network topology, and, thus, are relevant for interacting particle systems of all scales from neuronal networks, to power grids, to astrophysics. This talk is based on the joint work with Hayato Chiba (Tohoku University) and Matthew Mizuhara (The College of New Jersey).

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://us02web.zoom.us/j/84203723799?pwd=bmNVcVBaazRzSlI3TjAxV1U1S2xFQT09

Host
Freie Universität Berlin
WIAS Berlin
May 31 – June 2, 2021 (Online Event)
Workshop/Konferenz: SPDEs and their friends
more ... Location
Online Event

Host
WIAS Berlin
Technische Universität Berlin
Wednesday, 26.05.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Riccarda Rossi, Università degli Studi di Brescia, Italien:
Jump processes as generalized gradient systems (online talk)
more ... Location
Online Event

Abstract
The approach à la Jordan-Kinderlehrer-Otto to a vast class of evolutionary problems, interpreted as gradient flows in Wasserstein spaces, was extended by Maas and Mielke to Markov chains via the introduction of Benamou-Brenier type distances. Nonetheless, the study of the microscopic origin of jump processes by means of large deviations theory suggests that such processes possess a generalized gradient system structure based on non-homogeneous dissipation potentials that do not give rise to any metric structure. This talk revolves around the generalized gradient system structure that we have proposed for these processes in collaboration with Mark Peletier, Giuseppe Savaré and Oliver Tse. To build it, we have introduced a suitable `dynamical-variational' transport cost that induces a notion of length. Based on it, we can set forth an extended version of the Minimizing Movement scheme. We show the convergence of the discrete solutions arising from the time-incremental minimization scheme, to a curve fulfilling a suitable `non-metric' gradient flow formulation for the original jump process.

Further Informations
Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 26.05.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
András Tóbiás, TU Berlin:
Absence of percolation in graphs based on stationary point processes with degrees bounded by two (online talk)
more ... Location
Online Event

Abstract
We consider undirected graphs that arise as deterministic functions of stationary point processes such that each point has degree bounded by two. For a large class of point processes and edge-drawing rules, we show that the arising graph has no infinite connected component, almost surely. In particular, this extends our previous result for SINR graphs based on stabilizing Cox point processes and verifies the conjecture of Balister and Bollobás that the bidirectional k-nearest neighbor graph of a two-dimensional homogeneous Poisson point process does not percolate for k=2. The subject of this talk is joint work with B. Jahnel.

Further Informations
Seminar Interacting Random Systems (Online Event)

Host
WIAS Berlin
Wednesday, 26.05.2021, 09:00 (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Hans-Georg Müller, University of California, Davis, USA:
Functional models for time-varying random objects (online talk)
more ... Location
Online Event

Abstract
In recent years, samples of random objects and time-varying object data such as time-varying distributions or networks that are not in a vector space have become increasingly prevalent. Such data can be viewed as elements of a general metric space that lacks local or global linear structure. Common approaches that have been used with great success for the analysis of functional data, such as functional principal component analysis, are therefore not applicable. The concept of metric covariance makes it possible to define a metric auto-covariance function for a sample of random curves that take values in a general metric space and it can be shown to be non-negative definite when the squared semi-metric of the underlying space is of negative type. Then the eigenfunctions of the linear operator with the auto-covariance function as kernel can be used as building blocks for an object functional principal component analysis, which includes real-valued Frechet scores and metric-space valued object functional principal components. Sample based estimates of these quantities are shown to be asymptotically consistent and are illustrated with various data. (Joint work with Paromita Dubey, Stanford University.)

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Thursday, 20.05.2021, 14:00 (Online Event)
Seminar Numerische Mathematik
Prof. Thomas Richter, Universität Magdeburg:
Deep neural networks for accelerating fluid-dynamics simulations
more ... Location
Online Event

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 19.05.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Alexander Mielke, WIAS Berlin, Humboldt-Universität zu Berlin:
Towards a justification of GENERIC: coarse-graining of an infinite-dimensional Hamiltonian system (online talk)
more ... Location
Online Event

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 19.05.2021, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Hannes Leeb, University of Vienna, Österreich:
A (tight) upper bound for the length of confidence intervals with conditional coverage
more ... Location
Online Event

Abstract
We show that two popular selective inference procedures, namely data carving (Fithian et al., 2017) and selection with a randomized response (Tian et al., 2018b), when combined with the polyhedral method (Lee et al., 2016), result in confidence intervals whose length is bounded. This contrasts results for confidence intervals based on the polyhedral method alone, whose expected length is typically infinite (Kivaranovic and Leeb, 2020). Moreover, we show that these two procedures always dominate corresponding sample-splitting methods in terms of interval length.

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 12.05.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Thomas Eiter, WIAS Berlin:
Leray--Hopf solutions to a viscoelastic fluid model with nonsmooth stress-strain relation (online talk)
more ... Location
Online Event

Abstract
Subject of the talk is a fluid model including viscoelastic and viscoplastic effects. The state is given by the fluid velocity and an internal stress tensor that is transported along the flow with the Zaremba--Jaumann derivative. Moreover, the stress tensor obeys a nonlinear and nonsmooth dissipation law as well as stress diffusion. After a discussion of the model, the existence of global-in-time weak solutions, which satisfy an energy inequality, is shown under general Dirichlet conditions for the velocity field and Neumann conditions for the stress tensor.

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 12.05.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Tal Orenshtein, WIAS Berlin/TU Berlin:
Rough walks in random environment
more ... Location
Online Event

Abstract
Random walks in random environment (RWRE) have been extensively studied in the last half-century. Functional central limit theorems (FCLT) hold in some prototypical classes such the reversible and the ballistic ones. The latter are treated using rather different techniques; Kipnis-Varadhan's theory for additive functionals of Markov processes is applicable in the reversible case whereas the main feature exploited in the ballistic class is a regeneration structure. Rough path theory is a deterministic theory which extends classical notions of integration to singular integrators in a continuous manner. It typically provides a framework for pathwise solutions of ordinary and partial stochastic differential equations driven by a singular noise. In the talk we shall discuss FCLT for additive functionals of Markov processes and regenerative processes lifted to the rough path space. The limiting rough path has two levels. The first one is the Brownian motion, whereas in the second we see a new feature: it is the iterated integral of the Brownian motion perturbed by a deterministic linear function called the area anomaly. The aforementioned classes of RWRE are covered as special cases. The results provide sharper information on the limiting path. In addition, the construction of new examples for SDE approximations is an immediate application. Based on collaborations (some are still in progress) with Johannes Bäumler, Noam Berger, Jean-Dominique Deuschel, Olga Lopusanschi, Nicolas Perkowski and Martin Slowik.

Further Informations
Seminar Interacting Random Systems (Online Event)

Host
WIAS Berlin
Tuesday, 11.05.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Alicia Dickenstein, U Buenos Aires:
Algebra and geometry in the study of enzymatic networks? (online talk)
more ... Location
Online Event

Abstract
I will try to show in my lecture that the question in the title has a positive answer, summarizing recent mathematical results about signaling networks in cells obtained with algebro-geometric tools.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://us02web.zoom.us/j/84203723799?pwd=bmNVcVBaazRzSll3TjAxV1U1S2xFQT09.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 11.05.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Paul Hager, Technische Universität Berlin:
Optimal stopping with signatures (online talk)
more ... Location
Online Event

Abstract
We propose a new method for solving optimal stopping problems (such as American option pricing in finance) under minimal assumptions on the underlying stochastic process. We consider classic and randomized stopping times represented by linear and non-linear functionals of the rough path signature associated to the underlying process, and prove that maximizing over these classes of signature stopping times, in fact, solves the original optimal stopping problem. Using the algebraic properties of the signature, we can then recast the problem as a (deterministic) optimization problem depending only on the (truncated) expected signature. By applying a deep neural network approach to approximate the non-linear signature functionals, we can efficiently solve the optimal stopping problem numerically. The only assumption on the underlying process is that it is a continuous (geometric) random rough path. Hence, the theory encompasses processes such as fractional Brownian motion, which fail to be either semi-martingales or Markov processes, and can be used, in particular, for American-type option pricing in fractional models, e.g. on financial or electricity markets. This is a joint work with Christian Bayer, Sebastian Riedel and John Schoenmakers.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 04.05.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Dr. Maximilian Engel, FU Berlin:
Lyapunov exponents in random dynamical systems and how to find and use them
more ... Location
Online Event

Abstract
This talk aims to give an overview on various notions of Lyapunov exponents (LEs) in random dynamical systems, that is, systems whose evolution in time is governed by laws exhibiting randomness: from finite-time LEs to classical asymptotic LEs and corresponding spectra up to LEs for processes conditioned on staying in bounded domains. We demonstrate how these notions, especially of a first, dominant LE, become relevant in the context of stochastic bifurcations, in finite and infinite dimensions.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/84203723799.

Host
Freie Universität Berlin
WIAS Berlin
Monday, 03.05.2021, 15:00 (Online Event)
Berlin Oberseminar: Optimization, Control and Inverse Problems
Assoc. Prof. Lars Ruthotto, Emory University, USA:
A machine learning framework for mean field games and optimal control
more ... Location
Online Event

Abstract
We consider the numerical solution of mean field games and optimal control problems whose state space dimension is in the tens or hundreds. In this setting, most existing numercial solvers are affected by the curse of dimensionality (CoD). To mitigate the CoD, we present a machine learning framework that combines the approximation power of neural networks with the scalability of Lagrangian PDE solvers. Specifically, we parameterize the value function with a neural network and train its weight using the objective function with additional penalties that enforce the Hamilton Jacobi Bellman equations. A key benefit of this approach is that no training data is needed, e.g., no numerical solutions to the problem need to be computed before training. We illustrate our approach and its efficacy using numerical experiments. To show the framework's generality, we consider applications such as optimal transport, deep generative modeling, mean field games for crowd motion, and multi-agent optimal control.

Host
WIAS Berlin
April 29, 2021 (Online Event)
Workshop/Konferenz: FUHRI2021: Finite volUme metHods for Real-world applIcations
more ... Location
Online Event

Host
WIAS Berlin
Wednesday, 28.04.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Katharina Hopf, WIAS Berlin:
Weak-strong uniqueness in energy-reaction-diffusion systems (online talk)
more ... Location
Online Event

Abstract
We establish weak-strong uniqueness and stability properties of renormalised solutions to a class of energy-reaction-diffusion systems, which genuinely feature cross-diffusion effects. The systems considered are motivated by thermodynamically consistent models, and their formal entropy structure allows us to use as a key tool a suitably adjusted relative entropy method. Weak-strong uniqueness is obtained for general entropy-dissipating reactions without growth restrictions, and certain models with a non-integrable diffusive flux. The results also apply to a class of (isoenergetic) reaction-cross-diffusion systems.

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 28.04.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Alexander Hinsen, WIAS Berlin:
Percolation on a dynamical device-to-device communication system (online talk)
more ... Location
Online Event

Abstract
We model a system of moving devices on a street system with the help of a general waypoint kernel. If there is an uninterrupted communication window between devices (their space-time trajectories are close to each other) they form a connection. We study regimes for percolation and absence of percolation of the cluster of connected devices, as it can be seen as an indication for the ability of the system to transmit messages over long distances.

Further Informations
Seminar Interacting Random Systems (Online Event)

Host
WIAS Berlin
Tuesday, 27.04.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Eddie Nijholt, University of Illinois, USA:
Exotic symmetry in networks
more ... Location
Online Event

Abstract
Network dynamical systems appear all throughout science and engineering. Despite this prevalence, it remains unclear precisely how network structure impacts the dynamics. One very successful approach in answering this question is by identifying symmetry. Of course, there are many networks that do not have any form of symmetry, yet which still show remarkable dynamical behavior. Instead a wide array of other network features (such as node-dependency, synchrony spaces, and so forth) are known to impact the dynamics. We will see that most of these features can still be captured as symmetry, provided one widens the definition. That is, instead of considering classical group symmetry, one has to allow for more “exotic structures”, such as semigroups, categories and quivers. In many cases the network topology itself can even be seen as such a symmetry. An important consequence is that network structure can therefore be preserved in most reduction techniques, which in turn makes it possible to analyse bifurcations in such systems. In order to best explain these notions, l do not assume any familiarity with group symmetry -or their exotic counterparts- on the part of the audience.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/84203723799.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 27.04.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Peter Friz, WIAS Berlin, TU Berlin:
New perspectives on rough paths, signatures and signature cumulants (online talk)
more ... Location
Online Event

Abstract
We revisit rough paths and signatures from a geometric and “smooth model" perspective. This provides a lean framework to understand and formulate key concepts of the theory, including recent insights on higher-order translation a.k.a. renormalization of rough paths. This first part is joint work with C. Bellingeri (TU Berlin), and S. Paycha (U Potsdam). In a second part, we take a semimartingale perspective and more specifically analyze the structure of expected signatures when written in exponential form. Following Bonnier-Oberhauser (2020), we call the resulting objects signature cumulants. These can be described - and recursively computed - in a way that can be seen as unification of previously unrelated pieces of mathematics, including Magnus (1954), Lyons-Ni (2015), Gatheral and coworkers (2017 onwards) and Lacoin-Rhodes-Vargas (2019). This is joint work with P. Hager and N. Tapia.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 27.04.2021, 10:00 (Online Event)
Seminar Numerische Mathematik
Prof. Sashikumaar Ganesan, Indian Institute of Science Bangalore; Prof. Volker John, WIAS Berlin:
Minisymposium on machine learning
more ... Location
Online Event

Further Informations
For zoom login details please contact Volker John john@wias-berlin.de

Host
WIAS Berlin
Wednesday, 21.04.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Nikolas Nüsken, Universität Potsdam:
The Stein geometry in machine learning: gradient flows, large deviations and convergence properties (online talk)
more ... Location
Online Event

Abstract
Sampling or approximating high-dimensional probability distributions is a key challenge in computational statistics and machine learning. This talk will present connections to gradient flow PDEs and interacting particle systems, focusing on the recently introduced Stein variational gradient descent methodology. The construction induces a novel geometrical structure on the set of probability distributions related to a positive definite kernel function.We discuss the corresponding geodesic equations as well as large deviation functionals and leverage those to shed some light on the convergence properties of the algorithm. This is joint work with A. Duncan (Imperial College London), L. Szpruch (University of Edinburgh) and M. Renger (Weierstrass Institute Berlin).

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 20.04.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Alan Rendall, Johannes Gutenberg-Universität Mainz:
Bogdanov--Takens bifurcations and the regulation of enzymatic activity by autophosphorylation
more ... Location
Online Event

Abstract
An important mechanism of information storage in molecular biology is the binding of phosphate groups to proteins. In this talk we consider the case of autophosphorylation, where the protein is an enzyme and the substrate to which it catalyses the binding of a phosphate group is that enzyme itself. It turns out that this often leads to more complicated dynamics than those seen in the case where enzyme and substrate are distinct. We focus on the example of the enzyme Lck (lymphocyte-associated tyrosine kinase) which is of central importance in the function of immune cells. We study a model for the activation of Lck due to Kaimachnikov and Kholodenko and give a rigorous proof that it admits periodic solutions. We do so by showing that it exhibits a generic Bogdanov-Takens bifurcation. This is an example where this approach gives a simpler proof of the existence of periodic solutions than ones

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/84203723799.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 20.04.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Caroline Geiersbach, WIAS Berlin:
Stochastic approximation with applications to PDE-constrained optimization under uncertainty (online talk)
more ... Location
Online Event

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Thursday, 15.04.2021, 14:00 (Online Event)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Steven-Marian Stengl, WIAS Berlin:
Combined regularization and discretization of equilibrium problems and primal-dual gap estimators
more ... Location
Online Event

Abstract
In this talk, we address the treatment of finite element discretizations of a class of equilibrium problems involving moving constraints. Therefore, a Moreau-Yosida based regularization technique, controlled by a parameter, is discussed. A generalized Gamma-convergence concept is utilized to obtain a priori results. The same technique is applied to the discretization and the combination of both. In addition, a primal-dual gap technique is used for the derivation of error estimators and a strategy for balancing between a refinement of the mesh and an update of the regularization parameter is established. The theoretical findings are illustrated for the obstacle problem as well as numerical experiments are performed for two quasi-variational inequalities with application to thermoforming and biomedicine, respectively.

Further Informations
Seminar Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Thursday, 15.04.2021, 14:00 (Online Event)
Seminar Numerische Mathematik
Prof. Julia Novo, Universidad Autónoma de Madrid, Spanien:
Error analysis of proper orthogonal decomposition stabilized methods for incompressible flows
more ... Location
Online Event

Abstract
Proper orthogonal decomposition (POD) stabilized methods for the Navier-Stokes equations are presented. We consider two cases: the case in which the snapshots are based on a non inf-sup stable method and the case in which the snapshots are based on an inf-sup stable method. For both cases we construct approximations to the velocity and the pressure. For the first case, we analyze a method in which the snapshots are based on a stabilized scheme with equal order polynomials for the velocity and the pressure with local projection stabilization (LPS) for the gradient of the velocity and the pressure. For the POD method we add the same kind of LPS stabilization for the gradient of the velocity and the pressure as the direct method, together with grad-div stabilization. In the second case, the snapshots are based on an inf-sup stable Galerkin method with grad-div stabilization and for the POD model we also apply grad-div stabilization. In this case, since the snapshots are discretely divergence-free, the pressure can be removed from the formulation of the POD approximation to the velocity. To approximate the pressure, needed in many engineering applications, we use a supremizer pressure recovery method. Error bounds with constants independent of inverse powers of the viscosity parameter are proved for both methods. Numerical experiments show the accuracy and performance of the schemes.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 14.04.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Flaviana Iurlano, Sorbonne Université Paris, Frankreich:
Shape optimization of light structures (online talk)
more ... Location
Online Event

Abstract
We prove rigorous results about the vanishing-mass limit of the classical problem to find a shape with minimal elastic compliance. Contrary to all previous results in the mathematical literature, which utilize a soft mass constraint by introducing a Lagrange multiplier, we here consider the hard mass constraint. Our results are the first to establish the convergence of approximately optimal shapes of (exact) size $varepsilonto 0$ to a limit generalized shape represented by a (possibly diffuse) probability measure. This limit generalized shape is a minimizer of the limit compliance, which involves a new integrand, namely the one conjectured by Bouchitté in 2001 and predicted heuristically before in works of Allaire & Kohn and Kohn & Strang from the 1980s and 1990s. This integrand gives the energy of the limit generalized shape understood as a fine oscillation of (optimal) lower-dimensional structures. Its appearance is surprising since the integrand in the original compliance is just a quadratic form and the non-convexity of the problem is not immediately obvious. In fact, it is the interaction of the mass constraint with the requirement of attaining the loading (in the form of a divergence-constraint) that gives rise to this new integrand. Our proofs rest on compensated compactness arguments applied to an explicit family of div-quasiconvex quadratic forms, computations involving the Hashin--Shtrikman bounds for the Kohn--Strang integrand, and the characterization of limit minimizers due to Bouchitté & Buttazzo.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 14.04.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Alexis Prevost, University of Cambridge, GB:
Cluster capacity functionals and isomorphism theorems for Gaussian free fields (online talk)
more ... Location
Online Event

Abstract
Various isomorphisms theorem relate Gaussian free fields to random walk models, through their local times. Among them, the second Ray-Knight theorem can be extended in an isomorphism between the Gaussian free field and random interlacements on transient graphs. Following the work of Lupu and Sznitman, we will explain how the cable system method let us make this isomorphism more explicit, and how it relates to a certain cluster capacity functional for the Gaussian free field. Finally, we will present applications to percolation for the Gaussian free field on the cable system. Joint work with Alexander Drewitz (Cologne) and Pierre-François Rodriguez (London)

Further Informations
Seminar Interacting Random Systems (Online Event)

Host
WIAS Berlin
Tuesday, 13.04.2021, 16:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Yangwen Sun, Humboldt-Universität zu Berlin:
Graph-spanning ratio test with application to change-point detection problem (online talk)
more ... Location
Online Event

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 30.03.2021, 10:15 (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Simona Perotto, Politecnico di Milano, Italien:
Adaptive topology optimization in the design of structures and metamaterials (online talk)
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Host
WIAS Berlin
Monday, 29.03.2021, 15:00 (Online Event)
Berlin Oberseminar: Optimization, Control and Inverse Problems
Prof. Serge Gratton, ENSEEIHT, Toulouse, France:
On a multilevel Levenberg--Marquardt method for the training of artificial neural networks and its application to the solution of partial differential equations (online talk)
more ... Location
Online Event

Abstract
We propose a new multilevel Levenberg--Marquardt optimizer for the training of artificial neural networks with quadratic loss function. When the least-squares problem arises from the training of artificial neural networks, the variables subject to optimization are not related by any geometrical constraints and the standard interpolation and restriction operators cannot be employed any longer. A heuristic, inspired by algebraic multigrid methods, is then proposed to construct the multilevel transfer operators. We test the new optimizer on an important application: the approximate solution of partial differential equations by means of artificial neural networks. The learning problem is formulated as a least-squares problem, choosing the nonlinear residual of the equation as a loss function, whereas the multilevel method is employed as a training method. Numerical experiments show encouraging results related to the efficiency of the new multilevel optimization method compared to the corresponding one-level procedure in this context.

Further Informations
Seminar on Optimization, Control and Inverse Problems

Host
WIAS Berlin
Thursday, 25.03.2021, 14:00 (Online Event)
Seminar Numerische Mathematik
Benoît Gaudeul, Université de Lille, France:
Two entropic finite volume schemes for a Nernst--Planck--Poisson system with ion volume constraints
more ... Location
Online Event

Abstract
In this talk, I consider a drift-diffusion system with cross-coupling through the chemical potentials comprising a model for the motion of finite size ions in liquid electrolytes. The drift term is due to the self-consistent electric field maintained by the ions and described by a Poisson equation.
I present two finite volume schemes based on different formulations of the fluxes. I will also provide a stability analysis of these schemes and an existence result for the corresponding discrete solutions. A convergence result is proposed for non-degenerate solutions. Numerical experiments show the behavior of these schemes.
This is a joint work with Jürgen Fuhrmann.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Thursday, 18.03.2021, 14:00 (Online Event)
Seminar Numerische Mathematik
Prof. Leo Rebholz, Clemson University, USA:
Anderson acceleration and how it speeds up convergence in fixed point iterations
more ... Location
Online Event

Abstract
Anderson acceleration (AA) is an extrapolation technique originally proposed in 1965 that recombines the most recent iterates and update steps in a fixed point iteration to improve the convergence properties of the sequence. Despite being successfully used for many years to improve nonlinear solver behavior on a wide variety of problems, a theory that explains the often-observed accelerated convergence was lacking. In this talk, we give an introduction to AA, then present a proof of AA convergence which shows that it improves the linear convergence rate based on a gain factor of an underlying optimization problem, but also introduces higher order terms in the residual error bound. We then discuss improvements to AA based on our convergence theory, and show numerical results for the algorithms applied to several application problems including Navier--Stokes, Boussinesq, and nonlinear Helmholtz systems.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 17.03.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Daniel Heydecker, University of Cambridge, GB:
Large deviations of Kac's elastic particle system (online talk)
more ... Location
Online Event

Abstract
We consider Kac's stochastic model for a N-particle gas, with particles interacting by elastic collisions, and study the large deviations in the limit $Ntoinfty$ in the weak topology. There is a very natural candidate rate function, first proposed by Léonard, which amounts to the dynamic entropy of a flux measure plus an initial cost; with this rate function, we sketch the proof of an upper bound, and a lower bound restricted to a class of `sufficiently goods' paths. Perhaps surprisingly, we will see that the proposed rate function does not capture all of the possible large deviation behaviour. Lu and Wennberg showed that, even though the microscopic collisions preserve energy, there are solutions to the Boltzmann equation for which the energy is increasing, and these paths have 0 dynamic entropy. We will show that such paths can arise as large deviation limits with finite exponential cost, so cannot be excluded from large deviation analysis, but occur strictly more rarely than predicted by the rate function. At the level of the particle system, this occurs when a macroscopic proportion of the energy concentrates in $o(N)$ particles, which can happen with probability $e^-O(N)$.

Further Informations
Seminar Interacting Random Systems (Online Event)

Host
WIAS Berlin
Tuesday, 16.03.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Kostas Papafitsoros, WIAS Berlin:
Optimization with learning-informed differential equation constraints and its applications (online talk)
more ... Location
Online Event

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 16.03.2021, 13:30 (Online Event)
Seminar Numerische Mathematik
Dr. Jürgen Fuhrmann, WIAS Berlin:
PDELib.jl: Towards software components for the numerical solution of partial differential equations in Julia
more ... Location
Online Event

Abstract
In recent years, in particular since the release of version 1.0, the Julia programming language gained significant momentum in fields related to scientific computing and data science. Taking advantage of accumulated experience and know-how in language design, designed around the just-in-time compilation paradigm, and featuring first class multidimensional array handling, it allows for the implementation of complex numerical algorithms without sacrificing efficiency. In recent years, in particular since the release of version 1..0, the Julia programming language gained significant momentum in fields related to scientific computing and data science.
In the talk, we will give a short overview on features of the Julia language which renders it well suited for the implementation of solvers for complex systems of partial differential equations, including multiple dispatch allowing for the implementation of automatic differentiation, interface oriented API design and its package manager supporting reusability and reproducibility.
We will report on the successful steps towards the implementation of software components for the numerical solution of PDEs.. Focus will be on the package VoronoiFVM.jl and supporting packages.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Tuesday, 09.03.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Caroline Geiersbach, WIAS Berlin:
Stochastic approximation with applications to PDE-constrained optimization under uncertainty (online talk)
more ... Location
Online Event

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Wednesday, 03.03.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Cecile Mailler, The University of Bath, GB:
The ants walk: finding geodesics in graphs using reinforcement learning
more ... Location
Online Event

Abstract
How does a colony of ants find the shortest path between its nest and a source of food without any means of communication other than the pheromones each ant leave behind itself? In this joint work with Daniel Kious (Bath) and Bruno Schapira (Marseille), we introduce a new probabilistic model for this phenomenon. In this model, the nest and the source of food are two marked nodes in a finite graph. Ants perform successive random walks from the nest to the food, and ths distribution of the n-th walk depends on the trajectories of the (n-1) previous walks through some linear reinforcement mechanism. Using stochastic approximation methods, couplings with Pólya urns, and the electric conductances method for random walks on graphs, we prove that, in this model, the ants indeed eventually find the shortest path(s) between their nest and the source food.

Further Informations
Seminar Interacting Random Systems (Online Event)

Host
WIAS Berlin
Tuesday, 02.03.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Yunfei Huang, Ludwig-Maximilians-Universität München:
Advanced data analysis for traction force microscopy and data-driven discovery of physical equations (online talk)
more ... Location
Online Event

Abstract
The plummeting cost of collecting and storing data and the increasingly available computational power in the last decade have led to the emergence of new data analysis approaches in various scientific fields. Frequently, the new statistical methodology is employed for analyzing data involving incomplete or unknown information. In this thesis, new statistical approaches are developed for improving the accuracy of traction force microscopy (TFM) and data-driven discovery of physical equations. TFM is a versatile method for the reconstruction of a spatial image of the traction forces exerted by cells on elastic gel substrates. The traction force field is calculated from a linear mechanical model connecting the measured substrate displacements with the sought-for cell-generated stresses in real or Fourier space, which is an inverse and ill-posed problem. This inverse problem is commonly solved making use of regularization methods. Here, we systematically test the performance of new regularization methods and Bayesian inference for quantifying the parameter uncertainty in TFM. We compare two classical schemes, L1- and L2-regularization with three previously untested schemes, namely Elastic Net regularization, Proximal Gradient Lasso, and Proximal Gradient Elastic Net. We find that Elastic Net regularization, which combines L1 and L2 regularization, outperforms all other methods with regard to accuracy of traction reconstruction. Next, we develop two methods, Bayesian L2 regularization and Advanced Bayesian L2 regularization, for automatic, optimal L2 regularization.. We further combine the Bayesian L2 regularization with the computational speed of Fast Fourier Transform algorithms to develop a fully automated method for noise reduction and robust, standardized traction-force reconstruction that we call Bayesian Fourier transform traction cytometry (BFTTC). This method is made freely available as a software package with graphical user-interface for intuitive usage. Using synthetic data and experimental data, we show that these Bayesian methods enable robust reconstruction of traction without requiring a difficult selection of regularization parameters specifically for each data set. Next, we employ our methodology developed for the solution of inverse problems for automated, data-driven discovery of ordinary differential equations (ODEs), partial differential equations (PDEs), and stochastic differential equations (SDEs). To find the equations governing a measured time-dependent process, we construct dictionaries of non-linear candidate equations. These candidate equations are evaluated using the measured data. With this approach, one can construct a likelihood function for the candidate equations. Optimization yields a linear, inverse problem which is to be solved under a sparsity constraint. We combine Bayesian compressive sensing using Laplace priors with automated thresholding to develop a new approach, namely automatic threshold sparse Bayesian learning (ATSBL). ATSBL is a robust method to identify ODEs, PDEs, and SDEs involving Gaussian noise, which is also referred to as type I noise. We extensively test the method with synthetic datasets describing physical processes. For SDEs, we combine data-driven inference using ATSBL with a novel entropy-based heuristic for discarding data points with high uncertainty. Finally, we develop an automatic iterative sampling optimization technique akin to Umbrella sampling. Therewith, we demonstrate that data-driven inference of SDEs can be substantially improved through feedback during the inference process if the stochastic process under investigation can be manipulated either experimentally or in simulations.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Friday, 26.02.2021, 13:00 (Online Event)
AA2 - Materials, Light, Devices
Dr. Manuel Landstorfer, WIAS Berlin:
Modeling of concentration and electric field dependent susceptibilities in electrolytes (online talk)
more ... Location
Online Event

Host
Freie Universität Berlin
Humboldt-Universität zu Berlin
WIAS Berlin
Thursday, 25.02.2021, 14:00 (Online Event)
Seminar Numerische Mathematik
Vojtěch Miloš, University of Chemistry and Technology Technická, Prag; Petr Vágner, WIAS Berlin:
Modelling of YSZ|LSM|O_2 electrode with experimental validation
more ... Location
Online Event

Abstract
A generalized Poisson-Nernst-Planck model of an yttria-stabilized zirconia (YSZ) electrolyte endowed with a lanthanum strontium manganite (LSM)--oxygen electrode interface is formulated with in the framework of non-equilibrium thermodynamics. The model takes into account limitations in oxide ion concentrations due to the limited availability of oxygen vacancies. The electrolyte model is coupled with a general reaction kinetic scheme capturing the oxygen reduction.
The model is fitted and compared to a large experimental dataset of electrochemical impedance spectra and cyclic voltammograms spanning temperatures from 700$^circ$ C to 850$^circ$ C. An apriori temperature dependenciies of the model parameters are introduced and the Nelder-Mead algorithm is employed to minimize a compound cost function. The compound cost function allows for different weights if the two types of experiments leading to interesting physical interpretations.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 24.02.2021, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Dr. Torsten Hothorn, Universität Zürich, Switzerland:
Understanding and applying transformation models (online talk)
more ... Location
Online Event

Abstract
The mlt package implements maximum likelihood estimation in the class of conditional transformation models. Based on a suitable explicit parameterization of the unconditional or conditional transformation function using infrastructure from package basefun, we show how one can define, estimate, and compare a cascade of increasingly complex transformation models in the maximum likelihood framework. Models for the unconditional or conditional distribution function of any univariate response variable are set up and estimated in the same computational framework simply by choosing an appropriate transformation function and parameterization thereof. As it is computationally cheap to evaluate the distribution function, models can be estimated by maximization of the exact likelihood, especially in the presence of random censoring or truncation. The relatively dense high-level implementation in the R system for statistical computing allows generalization of many established implementations of linear transformation models, such as the Cox model or other parametric models for the analysis of survival or ordered categorical data, to the more complex situations illustrated in this talk.

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 23.02.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Prof. Dr. Alexandre Nolasco de Carvalho, University of São Paulo at São Carlos:
Gradient non-autonomous dynamics
more ... Location
Online Event

Abstract
In this lecture we present our approach for the study of the asymptotics of autonomous and non-autonomous dynamical systems. This unified approach shows how some different notions of attractors play a role in the description of the dynamics and enable us to address the gradient structure within non-autonomous attractors. As an application we characterize the gradient structure within the uniform attractor for a non-autonomous Chafee-Infante problem. A few, in our view, interesting and challenging problems, for future studies, are presented at the end of the lecture.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/82815629022.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 23.02.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Priv.-Doz. Dr. Christian Bayer, WIAS Berlin:
A pricing BSPDE for rough volatility (online talk)
more ... Location
Online Event

Abstract
In this talk, we study the option pricing problems for rough volatility models. As the framework is non-Markovian, the value function for a European option is not deterministic; rather, it is random and satisfies a backward stochastic partial differential equation (BSPDE). The existence and uniqueness of weak solution is proved for general nonlinear BSPDEs with unbounded random leading coefficients whose connections with certain forward-backward stochastic differential equations are derived as well. These BSPDEs are then used to approximate American option prices. A deep leaning-based method is also investigated for the numerical approximations to such BSPDEs and associated non-Markovian pricing problems. Finally, the examples of rough Bergomi type are numerically computed for both European and American options.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 23.02.2021, 10:15 (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Thomas Nagel, Technische Universität Berlin:
Surface detection of clouds by using simple artificial neural networks
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Host
WIAS Berlin
Thursday, 18.02.2021, 14:00 (Online Event)
Seminar Numerische Mathematik
Derk Frerichs, WIAS Berlin:
On reducing spurious oscillations in discontinuous Galerkin methods for convection-diffusion equations
more ... Location
Online Event

Abstract
Among other desirable features, standard discontinuous Galerkin methods for discretizing steady-state convection-diffusion-reaction equations are able to produce sharp layers in the convection-dominated regime, but also show large spurious oscillations. A computationally cheap way to reduce nonphysical oscillations is to apply post-processing techniques that replace the solution in the vicinity of layers by linear or constant approximations. This talk presents known post-processing methods form the literature, and proposes several generalizations as well as novel modifications. Results for two numerical benchmark problems are shown indicating pros and cons of the methods.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 17.02.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Giovanni P. Galdi, University of Pittsburgh, USA:
On the self-propelled motion of a rigid body in a viscous liquid by time-periodic boundary data (online talk)
more ... Location
Online Event

Abstract
Consider a rigid body, B, constrained to move by translational motion in an unbounded viscous liquid. The driving mechanism is a given distribution of time-periodic velocity field, v, at the interface body-liquid, of magnitude  (in appropriate function class). The main objective is to find conditions on v ensuring that B performs a non-zero net motion, namely, B can cover any given distance in a finite time. This study is mostly motivated by the self-propulsion of vibrating bodies in viscous liquids, where the vibration is generated by the oscillation of an internal mass. We show that the approach to the problem depends on whether the averaged value of v over a period of time is (case (b)) or is not (case (a)) identically zero. In case (a) we solve the problem in a relatively straightforward way, by showing that, for small , it reduces to the study of a suitable and well-investigated time-independent Stokes (linear) problem. In case (b), however, the question is much more complicated, because we show that it cannot be brought to the study of a linear problem. Therefore, in case (b), self-propulsion is a genuinely nonlinear issue that we solve directly on the nonlinear system by a contradiction argument. In this way, we are able to give, also in case (b), sufficient conditions for self-propulsion (for small ). Finally, we demonstrate, by means of counterexamples, that such conditions are, in general, also necessary.

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 17.02.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Thorsten Lucke, Technische Universität Berlin:
Influence of mobility on telecommunication networks --- some thoughts (online talk)
more ... Location
Online Event

Abstract
The Boolean model provides a nice framework for describing ad-hoc networks. It is well known that under certain assumptions on the distribution of the radii and the intensity of the Poisson process we can observe critical phenomena such as percolation or global connectivity. However, if we start in a subcritical regime, i.e., the intensity is below a critical threshold, can we manipulate the connectivity properties, if we allow the participants to move? This talk will provide a brief introduction to mobility models such as the random waypoint model.

Further Informations
Seminar Interacting Random Systems (Online Event)

Host
WIAS Berlin
Wednesday, 17.02.2021, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Georg Keilbar, Humboldt-Universität zu Berlin:
Finite sample bounds for quantiles using SGD (online talk)
more ... Location
Online Event

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 16.02.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Alexandre Pannier, Imperial College London , UK:
Large and moderate deviations for stochastic Volterra systems (online talk)
more ... Location
Online Event

Abstract
We provide a unified treatment of pathwise large and moderate deviations principles for a general class of multidimensional stochastic Volterra equations with singular kernels, not necessarily of convolution form. Our methodology is based on the weak convergence approach by Budhiraja, Dupuis and Ellis. We show in particular how this framework encompasses most rough volatility models used in mathematical finance, yields pathwise moderate deviations for the first time and generalises many recent results in the literature. This is joint work with Antoine Jacquier.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Friday, 12.02.2021, 13:00 (Online Event)
AA2 - Materials, Light, Devices
Dr. Benjamin Jurgelucks, Humboldt-Universität zu Berlin:
Material parameter identification and sensitivity optimization for piezoelectric ceramics (online talk)
more ... Location
Online Event

Abstract
Recent material parameter identification methods for piezoelectric ceramics rely on the solution of an inverse problem with the electrical impedance as the measurable quantity. A common complaint is that the sensitivity of impedance with respect to some material parameters is small or zero. Thus, some material parameters cannot be identified reliably. However, the sensitivity of impedance can be increased with the help of Algorithmic Differentiation by optimizing the shape of the electrodes attached to the ceramic. This greatly improves the accuracy of current material parameter identification methods for piezoelectrics. Furthermore, based on pre-optimized electrode shapes one can even achieve arbitrary sensitivity.

Further Informations
AA2 - Materials, Light, Devices

Host
Humboldt-Universität zu Berlin
Technische Universität Berlin
WIAS Berlin
Thursday, 11.02.2021, 14:00 (Online Event)
Seminar Numerische Mathematik
Dr. Ulrich Wilbrandt, WIAS Berlin:
Optimization of stabilization parameters for convection-dominated convection-diffusion equations
more ... Location
Online Event

Abstract
Solutions to convection-dominated convection-diffusion equations typically have spurious oscillations and other non-physical properties. To overcome these difficulties in the Finite Element context one often uses stabilizations, which are additional terms in the weak formulation. We will consider the streamline upwind Petrov-Galerkin (SUPG) and a spurious oscillations at layers diminishing (SOLD) method. These involve a user-chosen parameter on each cell and it is unknown how to choose these parameters exactly. The approach presented in this talk is to let an optimization algorithm decide on these parameters which leaves the choice of an appropriate objective to the user. We will show some objectives and their motivation and discuss a reduction of the rather larger control space.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 10.02.2021, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Dr. Eftychia Solea, École Nationale de la Statistique et de l'Analyse de l'Information (ENSAI), France:
Nonparametric and high-dimensional functional graphical models (online talk)
more ... Location
Online Event

Abstract
We consider the problem of constructing nonparametric undirected graphical models for high-dimensional functional data. Most existing statistical methods in this context assume either a Gaussian distribution on the vertices or linear conditional means. In this article we provide a more flexible model which relaxes the linearity assumption by replacing it by an arbitrary additive form. The use of functional principal components offers an estimation strategy that uses a group lasso penalty to estimate the relevant edges of the graph. We establish concentration inequalities for the resulting estimators allowing both the number of predictors and the number of functional principal components to diverge to infinity with increasing sample size. We also investigate the empirical performance of our method through simulation studies and a real data application.

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 09.02.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Svetlana Gurevich, Westfälische Wilhelms-Universität Münster:
Dynamics of temporal and spatio-temporal localized states in time-delayed systems
more ... Location
Online Event

Abstract
Time-delayed systems describe a large number of phenomena and exhibit a wealth of interesting dynamical regimes such as e.g., fronts, localized structures or chimera states. They naturally appear in situations where distant, pointwise, nonlinear nodes exchange information that propagates at a finite speed. In this talk, we review our recent theoretical results regarding the existence and the dynamics of temporal, spatial and spatio-temporal localized structures in the output of semiconductor mode-locked lasers. In particular, we discuss dispersive effects which are known to play a leading role in pattern formation. We show that they can appear naturally in delayed systems [1] and we exemplify our result by studying the influence of high order dispersion in a system composed of coupled optical microcavities.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/82815629022.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 09.02.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Franz Besold, WIAS Berlin:
Adaptive weights community detection (online talk)
more ... Location
Online Event

Abstract
We study a likelihood-based approach for the stochastic block model proposed by Kirill Efimov, Larisa Adamyan and Vladimir Spokoiny. We propose a modification of the algorithm to improve the rate of consistency.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Thursday, 04.02.2021, 16:00 (Online Event)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Shalva Amiranashvili, WIAS Berlin:
Parallel splitting methods
more ... Location
Online Event

Host
WIAS Berlin
Thursday, 04.02.2021, 15:00 (Online Event)
TES-Seminar on Energy-based Mathematical Methods and Thermodynamics
Prof. Dr. Eberhard Bänsch, Friedrich-Alexander Universität Erlangen-Nürnberg:
The Buongiorno model for nanofluids (online talk)
more ... Location
Online Event

Abstract
We present a mathematical model for convective transport in nanofluids including thermophoretic effects that is very similar to the celebrated model of Buongiorno. Our model, however, is thermodynamically consistent and consequently an energy estimate can be shown. We show existence of weak solutions for the time-dependent as well as stationary problem. Moreover, an efficient numerical scheme for the time dependent problem is proposed. For the stationary system, regularity and subsequently optimal error estimates for finite element approximations can be shown under some smallness assumptions.

Further Informations
TES-Seminar on Energy-based Mathematical Methods and Thermodynamics

Host
Technische Universität Berlin
WIAS Berlin
Wednesday, 03.02.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Petr Pelech, WIAS Berlin:
Separately global solutions to rate-independent systems - applications to large-strain deformations of damageable solids (online talk)
more ... Location
Online Event

Abstract
Rate-independent systems (RIS) are characterized by the lack of any internal time length scale: rescaling the input of the system in time leads to the very same rescaling of its solution. In continuum mechanics, rate-independent models represent a reasonable approximation whenever the external conditions change slowly enough so that the system can always reach its equilibrium. This applies if inertial, viscous, and thermal effects are neglected. RIS have proven to be useful in modeling of hysteresis, phase transitions in solids, elastoplasticity, damage, or fracture in small and large strain regimes. The talk introduces the notion of separately global solutions for RIS with non-convex functionals and describes an existence result for a model of bulk damage at large strains. The analysis covers non-convex energies blowing up for extreme compression, yields solutions excluding interpenetration of matter, and allows for handling nonlinear couplings of the deformation and the internal variable, which emerges e.g. from the interplay between Eulerian and Lagrangian description. It extends the theory developed so far in the small strain setting.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 03.02.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Elena Pulvirenti, Delft University of Technology, Niederlande:
The Widom-Rowlinson model: metastability, mesoscopic and microscopic fluctuations for the critical droplet (online talk)
more ... Location
Online Event

Abstract
We introduce the equilibrium Widom-Rowlinson model on a two-dimensional finite torus in which the energy of a particle configuration is attractive and determined by the union of small discs centered at the positions of the particles. We then discuss the metastable behaviour of a dynamic version of the WR model. This means that the particle configuration is viewed as a continuous time Markov process where particles are randomly created and annihilated as if the outside of the torus were an infinite reservoir with a given chemical potential. In particular, we start with the empty torus and are interested in the first time when the torus is fully covered by discs in the regime at low temperature and when the chemical potential is supercritical. In order to achieve the transition from empty to full, the system needs to create a sufficiently large droplet, called critical droplet, which triggers the crossover. We compute the distribution of the crossover time and identify the size and the shape of the critical droplet. The analysis relies on a mesoscopic and microscopic description of the surface of the critical droplet. It turns out that the critical droplet is close to a disc of a certain deterministic radius, with a boundary that is random and consists of a large number of small discs that stick out by a small distance. We will show how the analysis of surface fluctuations in the WR model allows us to derive the leading order term of the condensation time and also the correction order term. This is a joint work with Frank den Hollander (Leiden), Sabine Jansen (Munich) and Roman Kotecky (Prague & Warwick).

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Tuesday, 02.02.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
David Müller-Bender, Chemnitz University of Technology:
Laminar chaos in systems with time-varying delay
more ... Location
Online Event

Abstract
For many systems arising in nature and engineering, the influence of time-delays cannot be neglected. Due to environmental fluctuations that affect the delay generating processes such as transport processes, the delays are in general not constant but rather time-varying. Although it is known that variable delays can lead to interesting phenomena, their effect on the dynamics of systems is not completely understood. In this talk, it is demonstrated that a temporal delay variation can change the dynamics of a time-delay system drastically. A recently discovered type of chaos called laminar chaos is introduced, which can only be observed for a certain type of time-varying delays. It is characterized by nearly constant laminar phases with periodic duration, where the intensity of the laminar phases varies chaotically from phase to phase. In contrast to the high-dimensional turbulent chaos, which is also observed for constant delay, laminar chaos is low-dimensional. Furthermore, it is shown experimentally and theoretically that laminar chaos is a robust phenomenon. A time-series analysis toolbox for its detection is provided, which is benchmarked by experimental data and by time-series of a nonlinear delayed Langevin equation.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/82815629022.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 02.02.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Egor Gladin, Moscow Institute of Physics and Technology, Russische Föderation:
Solving convex min-min problems with smoothness and strong convexity in one variable group and small dimension of the other(online talk)
more ... Location
Online Event

Abstract
Logistic regression with the prior on one of the parameter groups is an example of the convex min-min problem with strong convexity in only one of the two variable groups. The talk is devoted to approaches to such problems that achieve linear convergence. The outer minimization problem is solved using Vaidya?s cutting plane method, and the inner problem (smooth and strongly convex) is solved using Nesterov's fast gradient method. Due to the importance of machine learning applications, we also consider the case when the objective function is a sum of a large number of functions. In this case, the variance-reduced accelerated gradient algorithm is used instead of Nesterov's fast gradient method.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 02.02.2021, 13:30 (Online Event)
Seminar Numerische Mathematik
Volker Kempf, Bundeswehr-Universität München:
Anisotropic and pressure-robust finite element discretizations for the Stokes equations
more ... Location
Online Event

Abstract
The incompressible Stokes equations show a fundamental invariance property, meaning that the velocity solution is not affected by a change of the right hand side data in form of a gradient field. Due to relaxation of the divergence constraint most common finite element discretizations, like the Taylor-Hood or Mini-element, are however not pressure-robust, i.e. they do not reproduce this property on the discrete level. The mentioned classical methods are also in general not inf-sup stable on anisotropic meshes with potentially unbounded aspect ratio, which are used e.g. when treating singularities caused by re-entrant edges.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Monday, 01.02.2021, 14:00 (Online Event)
Oberseminar Nonlinear Dynamics
Bernold Fiedler, Freie Universtität Berlin:
Global attractors of scalar parabolic equations: The Thom--Smale complex
more ... Location
Online Event

Abstract
We consider the global attractors of scalar parabolic equations ut=uxx+f(x,u,ux) under Neumann boundary conditions. The Thom-Smale complex decomposes a global attractor into the unstable manifolds of its (hyperbolic) equilibria ut=0. In general this is not a topological cell complex ? not even in the presence of a gradient structure. For the above PDEs, however, the Thom-Smale complex turns out to be a signed regular topological cell complex: the boundaries of the unstable manifolds are topological spheres, each with a signed hemisphere decomposition. On the other hand, the equilibria ut=0 are characterized by a meander curve, which arises from a shooting approach to the ODE boundary value problem uxx+f(x,u,ux) =0. We explore a minimax characterization of boundary neighbors, along the meander. Specifically, we identify the precise geometric locations of these boundary neighbors in the signed Thom-Smale complex. This opens an approach to the construction of global attractors with prescribed Thom-Smale complex. Many examples will illustrate this joint work with Carlos Rocha, dedicated to the memory of Geneviève Raugel. See arxiv:1811.04206 and doi: 10.1007/s10884-020-09836-5.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/82815629022.

Host
Freie Universität Berlin
WIAS Berlin
Monday, 01.02.2021, 13:00 (Online Event)
Oberseminar Nonlinear Dynamics
Carlos Rocha, Instituto Superior Técnico, Universidade de Lisboa, Portugal:
A minimax property for global attractors of scalar parabolic equations
more ... Location
Online Event

Abstract
We overview recent results on the geometric and combinatorial characterization of global attractors of semiflows generated by scalar semilinear partial parabolic differential equations under Neumann boundary conditions. In special, we discuss a minimax property of the boundary neighbors of any specific unstable equilibrium. This property allows the identification of the equilibria on the cell boundary of any chosen equilibrium. This is a joint work with B. Fiedler.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/82815629022.

Host
Freie Universität Berlin
WIAS Berlin
Thursday, 28.01.2021, 15:00 (Online Event)
TES-Seminar on Energy-based Mathematical Methods and Thermodynamics
Ass. Prof. Markus Hütter, Eindhoven University of Technology, Niederlande:
Finite-size effects in complex fluids: Phenomenology of fluctuations, stochastics, and statistical mechanics (online talk)
more ... Location
Online Event

Abstract
In general, in every system with dissipation there are also fluctuations; the fluctuation-dissipation theorem is a key result of nonequilibrium statistical mechanics, which is encoded e.g. in the GENERIC framework [1,2,3]. Correspondingly, also complex fluids, which contain the relaxation of the microstructure (e.g. polymer chains), must show behavior with fluctuations. Fluctuations are relevant particularly for the behavior on small scales, where they are not averaged out, e.g. in micro-/nano-fluidics and microrheology. To date, models for complex fluids do not include fluctuations systematically. In our recent work, we have extended conformation-tensor based models for complex fluids by including thermal fluctuations in a thermodynamically consistent way [4]. Furthermore, this work has been complemented by establishing a link between the fluctuating conformation-tensor models and the underlying description of polymer chains as bead-spring dumbbells [5]. To achieve this link, we follow two routes: (1) stochastic calculus, followed by averaging (direct approach); (2) nonequilibrium statistical mechanics, which includes the derivation of the coarse-grained relaxation tensor, the fluctuations, and the thermodynamic potential (particularly entropy). Denoting the number of polymer chains by N, it is shown that a finite (rather than infinite) value for N gives rise to a correction to the conventional entropy for the conformation tensor and leads to the fluctuations that vanish only in the thermodynamic limit (N to infinity). Finally, some subtleties are pointed out related to the appearance of the so-called spurious drift in the stochastic differential equation for the conformation tensor. References: [1] M. Grmela, H.C. Öttinger, Phys. Rev. E 56(6) (1997) 6620?6632 (http://dx.doi.org/10.1103/PhysRevE.56.6620) [2] H.C. Öttinger, M. Grmela, Phys. Rev. E 56(6) (1997) 6633?6655 (http://dx.doi.org/10.1103/PhysRevE.56.6633) [3] H.C. Öttinger, Beyond Equilibrium Thermodynamics, Wiley, Hoboken, 2005. [4] M. Hütter, M.A. Hulsen, P.D. Anderson, J. Non-Newtonian Fluid Mech. 256 (2018) 42?56 (https://doi.org/10.1016/j.jnnfm.2018.02.012) [5] M. Hütter, P.D. Olmsted, D.J. Read, Eur. Phys. J. E 43 (2020) 71 (http://dx.doi.org/10.1140/epje/i2020-11999-x)

Further Informations
TES-Seminar on Energy-based Mathematical Methods and Thermodynamics

Host
Technische Universität Berlin
WIAS Berlin
Wednesday, 27.01.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Sebastian Throm, Westfälische Wilhelms-Universität Münster:
Universal long-time asymptotics for coagulation equations (online talk)
more ... Location
Online Event

Abstract
In this talk, we will consider the long-time behaviour of coagulating systems. Clustering phenomenon can be observed in many processes in nature and industry and also on a broad range of scales. Our focus will be on Smoluchowski's coagulation equation and the question of universal scaling for large times - a problem which is known as scaling hypothesis. In particular, we will present a recent result proving this conjecture for perturbations of a constant rate kernel.

Further Informations
Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 26.01.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Tibor Krisztin, University of Szeged, Hungary:
Periodic and connecting orbits for the Mackey--Glass equation
more ... Location
Online Event

Abstract
We consider the Mackey-Glass equation x' (t)=-ax(t)+b (x^k(t-1))/(1+x^n (t-1)) with positive parameters a,b,k,n. First, for the discontinuous limiting (n??) case, an orbitally asymptotically stable periodic orbit is constructed for some fixed parameter values a,b,k. Then it is shown that for large values of n and with the same parameters a,b,k, the Mackey-Glass equation also has an orbitally asymptotically stable periodic orbit near to the periodic orbit of the discontinuous equation. The shape of the obtained stable periodic solutions can be complicated. The existence of connecting orbits will be discussed as well.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/82815629022.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 26.01.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Priv.-Doz. Dr. John Schoenmakers, WIAS Berlin:
From optimal martingales to randomized dual optimal stopping
more ... Location
Online Event

Abstract
In this talk we study and classify dual optimal martingales in the dualformulation of optimal stopping problems. In this respect it is distinguished between optimal and surely optimal martingales. It is shown that the family of optimal and surely optimal martingales may generally be quite large. On the other hand it is shown that the Doob-martingale, i.e. the martingale part of the Snell envelope, is in a certain sense the most robust surely optimal martingale under random perturbations. This new insight has led to a novel randomized dual martingale minimization algorithm that doesn't require nested simulation. As a main feature, in a possibly large family of optimal martingales the algorithm sorts out efficiently a martingale that is close to the Doob martingale. As a result, one obtains in an efficient way a dual estimate for the optimal stopping problem with low variance. (joint work with Denis Belomestny)

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Thursday, 21.01.2021, 15:00 (Online Event)
TES-Seminar on Energy-based Mathematical Methods and Thermodynamics
Dr. Pierre-Étienne Druet, Technische Universität Berlin WIAS Berlin:
The free energy of incompressible fluid mixtures: An asymptotic study (online talk)
more ... Location
Online Event

Abstract
The results presented in this talk are triggered by the modelling of multicomponent fluids at low Mach-number. We first perform the asymptotic of small compressibility for a globally stable phase, and we show that the Helmholtz free energy, which is the crucial constitutive quantity, tends to a limit in the sense of epi- or Gamma-convergence. The essential findings are: The limit model does not only prohibit thermal expansion - this is the well-known Mueller-paradox -, but it also excludes the phenomena of volume loss or excess due to mixing. In order to temper the latter unrealistic conclusion, we modify the basic scenario by performing the limit only inside of a bounded, stable phase. In this way we recover incompressible systems that are not volume additive. Moreover, the limit free energy depends continuously on the state variables. This research is joined work with D.Bothe (TU Darmstadt) and W.Dreyer (WIAS Berlin).

Further Informations
TES-Seminar on Energy-based Mathematical Methods and Thermodynamics (online talk)

Host
Technische Universität Berlin
WIAS Berlin
Thursday, 21.01.2021, 14:00 (Online Event)
Seminar Numerische Mathematik
Felipe Galarce, INRIA, Paris:
Inverse problems in haemodynamics -- fast estimation of blood flows from medical data
more ... Location
Online Event

Abstract
We present a study at the interface between applied mathematics and biomedical engineering. The work's main subject is the estimation of blood flows and quantities of medical interest in diagnosing certain diseases concerning the cardiovascular system. We propose a complete pipeline, providing the theoretical foundations for state estimation from medical data using reduced-order models, and addressing inter-patient variability. Extensive numerical tests are shown in realistic 3D scenarios that verify the potential impact of the work in the medical community.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 20.01.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Lorenzo Taggi, University of Rome “La Sapienza”, Italien:
Exponential decay of correlations for O(N) spin systems
more ... Location
Online Event

Abstract
The Spin O(N) model is a classical statistical mechanics model whose configurations are collections of unit vectors, called spins, taking values on the surface of a N ?1 dimensional unit sphere, with each spin associated to the vertex of a graph. Some special cases of the spin O(N) model are the Ising model (N = 1), the XY model (N = 2), and the classical Heisenberg model (N = 3). Despite the fact that it is a very classical model, there remain important gaps in understanding, particularly in the case N > 2. This talk will provide an introduction on this class of models and present a new recent result about exponential decay of correlations for arbitrary (non-zero) values of the external magnetic field and arbitrary spin dimension N>1. The proof of this result employs a representation of the model as a system of coloured random paths and a sampling procedure, which allows us to bound from above the "typical lenght" of the open paths. The talk is based on a joint work with B. Lees.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Wednesday, 20.01.2021, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Sven Wang, University of Cambridge:
On polynomial-time computation of high-dimensional posterior measures by Langevin-type algorithms
more ... Location
Online Event

Abstract
In this talk, we consider the task of generating random samples of high-dimensional posterior distributions. The main results consist of non-asymptotic computational guarantees for Langevin-type MCMC algorithms which scale polynomially in key quantities such as the dimension of the model, the desired precision level, and the number of statistical measurements. We also show that posterior mean vectors and maximum a posteriori (MAP) estimates are computable in polynomial time, with high probability under the distribution of the data. Those results are derived in a general non-linear regression setting where posterior measures are not necessarily log-concave. The main example is a non-linear PDE model involving the steady-state Schrödinger equation. The talk is based on the preprint https://arxiv.org/abs/2009.05298.

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 19.01.2021, 16:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Dmitrii M. Ostrovskii, University of Southern California:
Near-optimal model discrimination with non-disclosure (online talk)
more ... Location
Online Event

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 19.01.2021, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Dr. Michal Hadrava, Institute of Computer Science of the Czech Academy of Sciences, Prague, Czech Republic:
A dynamical systems approach to spectral music: Modeling the role of roughness and inharmonicity in perception of musical tension
more ... Location
Online Event

Abstract
Tension-resolution patterns seem to play a dominant role in shaping our emotional experience of music. Whereas in traditional Western music, these patterns are mainly expressed through harmony and melody, many contemporary musical compositions (e.g. so-called ?spectral music") employ sound materials lacking any perceivable pitch structure, rendering these two compositional devices useless. Motivated by recent advances in music-theoretical and neuroscientific research into the related phenomenon of dissonance, we propose a neurodynamical model of musical tension based on a spectral representation of sound and hence applicable to any kind of sound material, pitched or non-pitched.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/82815629022.

Host
Freie Universität Berlin
WIAS Berlin
Thursday, 14.01.2021, 16:00 (Online Event)
Forschungsseminar Mathematische Modelle der Photonik
Rostislav Arkhipov, St. Petersburg State University:
Dissipative self-induced transparency solitons and rogue waves in coherent mode-locking. Unipolar light
more ... Location
Online Event

Host
WIAS Berlin
Thursday, 14.01.2021, 15:00 (Online Event)
TES-Seminar on Energy-based Mathematical Methods and Thermodynamics
Dr. Denis Matignon, Université Fédérale Toulouse Midi-Pyrénées, Frankreich:
The Partitioned Finite Element Method for port-Hamiltonian systems: structure-preserving numerics for physics-based PDEs with boundary control (online talk)
more ... Location
Online Event

Abstract
The numerical simulation of complex open multiphysics systems in Computational Science and Engineering is a challenging topic. Based on energy exchanges, the port-Hamiltonian formalism aims at describing physics in a structured manner. One of the major interests of this approach is its versatility, allowing for coupling and interconnection that preserve this structure. We propose a Finite Element based technique for the structure-preserving discretization of a large class of port-Hamiltonian systems (pHs). Assuming a partitioned structure of the system associated to an integration-by-parts formula, it is possible to derive a consistent weak-formulation sharing the main features of the original boundary-controlled PDE. This allows using Galerkin approximations to obtain finite-dimensional systems that mimic the properties of the original distributed ones; these systems are either ODEs or Differential Algebraic Equations (DAEs). Moreover, the Partitioned Finite Element Method producing sparse matrices enables the use of dedicated algorithms in scientific computing. Indeed, this method can be easily implemented using well-established and robust libraries. This strategy is illustrated by means of physically motivated PDEs with boundary control and observation, either in 2D or in 3D, both linear and non-linear: acoustic waves, Mindlin and Kirchhoff plates, heat equation, Shallow Water equation, Maxwell's equation. Interactive Jupyter notebooks are available, relying on the FEniCS open-source software. Advanced applications include multiphysics problems, e.g. fluid-structure interactions, thermoelasticity, and modular modelling of complex systems, e.g. multibody dynamics.

Further Informations
TES-Seminar on Energy-based Mathematical Methods and Thermodynamics

Host
Technische Universität Berlin
WIAS Berlin
Wednesday, 13.01.2021, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Anieza Maltsi, WIAS Berlin:
On the Darwin--Howie--Whelan equations for the scattering of fast electrons described by the Schrödinger equation (online talk)
more ... Location
Online Event

Abstract
The Darwin--Howie--Whelan (DHW) equations are widely used for the numerical simu-lation of transmission electron microscopy (TEM) images. They describe the propagation of the electron beam through crystalline solids and are formally stated as a system of infinitely many beam amplitudes?g. For the numerical simulation however only a finite set of beams is used, e.g. the two-beams approximation. In this talk we will address the accuracy of the approximation of this infinite system by suitable finite-dimensional ones.

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 13.01.2021, 11:30 (Online Event)
Seminar Interacting Random Systems
Sabine Jansen, Universität München:
Virial inversion and density functionals (online talk)
more ... Location
Online Event

Abstract
We present a novel inversion theorem for functionals that map measures to measures. Our main application is in statistical mechanics of inhomogeneous systems, in which the activity $z(x) = z_0 exp( - V_ext(x))$ is mapped to a density profile $rho(x)$ and one seeks to invert the density-activity relation. In probabilistic terms, the question is how to invert the relation between the intensity measure of an underlying Poisson point process and the intensity measure of a Gibbs point process, a question also of interest in spatial statistics. We apply the inversion theorem to the derivation of density functionals. The inversion theorem works in situations where the inverse function theorem in Banach spaces fails. Based on joint work with Tobias Kuna and Dimitrios Tsagkarogiannis (arXiv:1906.02322 [math-ph]).

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Wednesday, 13.01.2021, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Dr. Vladimir Spokoiny, WIAS Berlin:
Bayesian inference in machine learning (online talk)
more ... Location
Online Event

Abstract
Statistical inference for binary data is one of the central problem in machine learning which can be treated within a nonparametric Bernoulli model. In many situations the underlying Bernoulli parameter may approach the edges zero or one that leads to inference for non-regular models. Properties like asymptotic normality of the MLE or of the posterior distribution are not valid in such irregular cases. The approach of this paper suggests to impose a Gaussian prior on the canonical Bernoulli parameter, or, analogously, to consider a penalized MLE with a quadratic penalty. A Gaussian prior can be viewed as a kind of a barrier preventing the canonical parameter to approach infinity and forcing it to stay within the region of regularity. The main results stay the classical results like asymptotic normality of the posterior distribution or asymptotic normality of the penalized MLE for any configuration of Bernoulli parameters under the assumption that the prior is strong enough in direction. We also demonstrate how the results can be applied to analysis of random graphs, ranking problems, and nonparametric binary classification.

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 12.01.2021, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Pavel Dvurechensky, WIAS Berlin:
Accelerated alternating minimization methods (online talk)
more ... Location
Online Event

Abstract
We combine alternating minimization (AM) and Nesterov-type momentum acceleration and propose a generic accelerated alternating minimization method with a $1/k^2$ convergence rate in terms of the objective for convex problems and $1/k$ in terms of the squared gradient norm for non-convex problems, where $k$ is the iteration counter. Our method does not require any knowledge of neither convexity of the problem nor function parameters such as smoothness constant, i.e. it is adaptive to convexity and smoothness. Further, we develop its primal-dual modification for convex problems with linear constraints. We consider two applications of our methods to highlight their properties. The first one is the non-convex collaborative filtering problem, and the second one is optimal transport, where our primal-dual method takes a form of accelerated Sinkhorn's algorithm or accelerated Iterative Bregman Projections algorithm. In both applications, we show numerically that our methods outperform the baselines. In the second application, we also obtain state-of-the-art complexity results for optimal transport problems.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
January 8, 2021 (Online Event)
Workshop/Konferenz: Applied Probability Day
more ... Location
Online Event

Further Informations
Der Workshop findet bei Zoom statt: https://tu-berlin.zoom.us/j/69959389549?pwd=YWtnam1OSXNRRnVoWVJtUHJWdFpzZz09

Host
WIAS Berlin
Institute of Mathematical Statistics
Wednesday, 06.01.2021, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Alexey Naumov, HSE University, Russische Föderation:
Finite time analysis of linear two-timescale stochastic approximation with Markovian noise (online talk)
more ... Location
Online Event

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Thursday, 17.12.2020, 16:00 (Online Event)
Forschungsseminar Mathematische Modelle der Photonik
Max Schiemangk, FBH Berlin:
Requirements on lasers for optical quantum metrology
more ... Location
Online Event

Host
WIAS Berlin
Wednesday, 16.12.2020, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Howard Bondell, University of Melbourne, Australien:
Bayesian regression for high-dimensional data using a prior on the model fit
more ... Location
Online Event

Abstract
We propose a new class of prior distributions for shrinkage regularization in sparse linear regression, particularly the high dimensional case. Instead of placing a prior on the coefficients themselves, we place a prior on the regression R-squared. This is then distributed to the coefficients in a suitable manner. We show that this framework induces a shrinkage prior that can obtain desirable theoretical properties. Compared to existing shrinkage priors, it can simultaneously achieve both high prior concentration at zero, as well as heavier tails. These two properties combine to provide a higher degree of shrinkage on the irrelevant coefficients, along with less bias in estimation of the larger signals. Both theoretical and practical aspects will be discussed.

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Friday, 11.12.2020, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Judith Rousseau, University of Oxford, UK:
Bayesian nonparametric estimation in multivariate non linear Hawkes processes (online talk)
more ... Location
Online Event

Further Informations
Der Vortrag findet bei Zoom statt: https://uni-potsdam.zoom.us/j/61627954803

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Thursday, 10.12.2020, 15:00 (Online Event)
TES-Seminar on Energy-based Mathematical Methods and Thermodynamics
Dr. Martin Kružík, Czech Academy of Sciences:
Equilibrium for multiphase solids with Eulerian interfaces (online talk)
more ... Location
Online Event

Abstract
We describe a general phase-field model for hyperelastic multiphase materials. The model features an elastic energy functional that depends on the phase-field variable and a surface energy term that depends in turn on the elastic deformation, as it measures interfaces in the deformed configuration. We prove the existence of energy minimizing equilibrium states and Gamma-convergence of diffuse-interface approximations to the sharp-interface limit. This is a joint work with D. Grandi, E. Mainini and U. Stefanelli.

Further Informations
Please register on this link: https://mathplus.de/topic-development-lab/tes-winter-2020-21/tes-seminar-on-energy-based-mathematical-methods-and-thermodynamics/

Host
Technische Universität Berlin
WIAS Berlin
Wednesday, 09.12.2020, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Grigor Nika, WIAS Berlin:
Effective bulk-surface thermistor models for OLEDs (online talk)
more ... Location
Online Event

Abstract
We consider a thin-film OLED device placed on a flat surface, part of a bulk substrate material. The device is assumed to have N>2 layers and total thickness h>0. An effective electrothermal model is derived by considering the limit of vanishing thickness as it approaches zero under physically motivated scalings. The effective equations consist of two equations for the lateral current flow in the top and bottom electrodes and an iterative set of algebraic equations for the vertical flow in the organic layers posed on the flat surface where the OLED is placed and coupled to the heat equation in the bulk substrate material.

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)

Host
WIAS Berlin
Wednesday, 09.12.2020, 11:30 (Online Event)
Seminar Interacting Random Systems
Robert Patterson:
Large deviations for population growth from near zero concentration
more ... Location
Online Event

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Tuesday, 08.12.2020, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Elisenda Feliu, University of Copenhagen, Denmark:
Understanding of bistability and Hopf bifurcations in biochemical reaction networks
more ... Location
Online Event

Abstract
In the context of (bio)chemical reaction networks, the dynamics of the concentrations of the chemical species over time are often modelled by a system of parameter-dependent ordinary differential equations, which are typically polynomial or described by rational functions. The polynomial structure of the system allows the use of techniques from algebra to study properties of the system around steady states, for arbitrary parameter values. In this talk I will present the formalism of the theory of reaction networks, and how applied algebra plays a role in the study of three main questions: determination of bistability, determination of Hopf bifurcations, and parameter regions for multistationarity. I will present new results tackling these questions by using a ubiquitous and challenging network from cell signaling (the dual futile cycle) as a case example. For this network, which is relatively small, several basic questions, such as the existence of oscillations, the parameter region of multistationarity, and whether multistationarity implies bistability, remain unresolved.

Host
Freie Universität Berlin
WIAS Berlin
Wednesday, 02.12.2020, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Dr. Yegor Klochkov, University of Cambridge, UK:
Robust K-means clustering under two moments
more ... Location
Online Event

Abstract
We consider the robust algorithms for the k-means clustering problem where a quantizer is constructed based on N independent observations. Our main results are median of means based non-asymptotic excess distortion bounds that hold under the two bounded moments assumption in a general separable Hilbert space. In particular, our results extend the renowned asymptotic result of Pollard(1981) who showed that the existence of two moments is sufficient for strong consistency of an empirically optimal quantizer in R^d. In a special case of clustering in R^d, under two bounded moments, we prove matching (up to constant factors) non-asymptotic upper and lower bounds on the excess distortion, which depend on the probability mass of the lightest cluster of an optimal quantizer. Our bounds have the sub-Gaussian form, and the proofs are based on the versions of uniform bounds for robust mean estimators. This is joint work with Alexei Kroshnin and Nikita Zhivotovskiy.

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
December 2 – 3, 2020 (Online Event)
Workshop/Konferenz: ECMI Webinar “Math for Industry 4.0 - Models, Methods and Big Data”
more ... Location
Online Event

Host
WIAS Berlin
Tuesday, 01.12.2020, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Dr. Felix Kemeth, Johns Hopkins University, USA:
2-cluster fixed-point analysis of mean-coupled Stuart--Landau oscillators in the center manifold
more ... Location
Online Event

Abstract
Clustering in phase oscillator systems with long-range interactions has been subject to theoretical investigations for many years. By mapping the dynamics of mean-coupled Stuart-Landau oscillators onto the center manifold of the Benjamin-Feir instability, we aim to add to the theoretical understanding of clustering in systems beyond phase oscillator ensembles. In particular, we discuss the formation of 2-cluster states in this lower dimensional manifold and outline the resulting implications for the dynamics of the coupled oscillator ensemble.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 01.12.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Roman Kravchenko, Humboldt-Universität zu Berlin:
Distributed optimization with quantization for computing Wasserstein barycenters
more ... Location
Online Event

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Thursday, 26.11.2020, 16:00 (Online Event)
Forschungsseminar Mathematische Modelle der Photonik
Regina Finsterhölzl, TU Berlin:
Applying feedback control on quantum many-body systems within the matrix-product-state framework
more ... Location
Online Event

Host
WIAS Berlin
Thursday, 26.11.2020, 15:00 (Online Event)
TES-Seminar on Energy-based Mathematical Methods and Thermodynamics
Prof. Dr. Miroslav Grmela, Polytechnique Montréal:
Multiscale thermodynamics (online talk)
more ... Location
Online Event

Abstract
Multiscale thermodynamics is a theory of relations among levels of investigation of complex systems. It includes the classical equilibrium thermodynamics as a special case but it is applicable to both static and time evolving processes in externally and internally driven macroscopic systems that are far from equilibrium and are investigated on microscopic, mesoscopic, and macroscopic levels. In this talk we formulate the multiscale thermodynamics, explain its origin, and illustrate it in mesoscopic dynamics that combines levels, see [1] for more details. [1] M. Grmela: Multiscale Thermodynamics. arxiv-preprint 2020.

Further Informations
Please register on this link: https://mathplus.de/topic-development-lab/tes-winter-2020-21/tes-seminar-on-energy-based-mathematical-methods-and-thermodynamics/

Host
Technische Universität Berlin
WIAS Berlin
Wednesday, 25.11.2020, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Annika Bach, Technische Universität München:
Geometrically frustrated spin systems: The antiferromagnetic XY model on the triangular lattice (online talk)
more ... Location
Online Event

Abstract
Antiferromagnetic spin systems are magnetic lattice systems in which the exchange interaction between two spins (i.e., vectors in S1) favors anti-alignment. Such systems are said to be geometrically frustrated if, due to the geometry of the lattice, there is no orientation of spins that simultaneously minimizes all pairwise interactions. This is the case for the antiferromagnetic XY model on the two-dimensional triangular lattice. As a consequence, the system has two families of ground states which can be distinguished one from the other by what is called their chirality. In this talk we present a recent result obtained in collaboration with M. Cicales, L. Kreutz, and G. Orlando, where we characterise the discrete-to-continuum Gamma-limit of the XY-model energy in a regime which detects chirality transitions on one-dimensional interfaces between the two admissible chirality phases.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 25.11.2020, 11:30 (Online Event)
Seminar Interacting Random Systems
Willem van Zuijlen:
Total mass asymptotics of the parabolic Anderson model
more ... Location
Online Event

Abstract
We consider the parabolic Anderson model with a white noise potential in two dimensions. This model is also called the stochastic heat equation with a multiplicative noise. We study the large time asymptotics of the total mass of the solution. Due to the irregularity of the white noise, in two dimensions the equation is a priori not well-posed. Using paracontrolled calculus or regularity structures one can make sense of the equation by a renormalisation, which can be thought of as “subtracting infinity of the potential”. To obtain the asymptotics of the total mass we use the spectral decomposition, an alternative Feynman-Kac type representation and heat-kernel estimates which come from joint works with Khalil Chouk, Wolfgang König and Nicolas Perkowski.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Wednesday, 25.11.2020, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Dr. Alexander Kreiß, KU Leuven:
Unbiased Markov chain Monte Carlo methods with couplings
more ... Location
Online Event

Abstract
We will consider multivariate stochastic processes indexed either by vertices or pairs of vertices of a dynamic network. Under a dynamic network we understand a network with a fixed vertex set and an edge set which changes randomly over time. We will assume that the spatial dependence-structure of the processes conditional on the network behaves in the following way: Close vertices (or pairs of vertices) are dependent, while we assume that the dependence decreases conditionally on that the distance in the network increases. We make this intuition mathematically precise by considering three concepts based on correlation, beta-mixing with time-varying beta-coefficients and conditional independence. These concepts allow proving weak-dependence results, e.g. an exponential inequality. These results will be useful for studying asymptotic regimes in which the network grows (i.e. the number of vertices tends to infinity) and we can expect that a growing network results in a larger distance between most vertices. Since the network is dynamic we allow that vertices move around the network and come close to different vertices. In order to demonstrate the use of these concepts in an application we study the asymptotics (for growing networks) of a goodness of fit test in a dynamic interaction network model based on a Cox-type model for counting processes. This model is then applied to bike-sharing data.

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 24.11.2020, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Dr. Giulia Ruzzene, Universitat Pompeu Fabra, Palma de Mallorca, Spain:
Control of chimera states in multilayer networks
more ... Location
Online Event

Abstract
Chimera states are one of the most intriguing and studied types of partial synchronization. In small systems, which are the most relevant for experimental situations, chimera states present various instabilities. Therefore, it is natural to investigate methods to control them. We propose a control mechanism based on the idea of a pacemaker oscillator, which allows to control the position of a chimera state within a network and to prevent its collapse to the fully synchronous state. We show how this mechanism developed for ring networks of phase oscillators can be applied to multilayer networks with more complex node dynamics, such as FitzHugh-Nagumo oscillator. In particular, we show that it allows to remotely control a chimera state in one layer via a pacemaker in the other layer.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/82815629022.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 24.11.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexander Gasnikov, WIAS Berlin:
Parallel and distributed algorithms for ML problems
more ... Location
Online Event

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 24.11.2020, 10:15 (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Sergey Nechayev, Ferdinand-Braun-Institut, Berlin:
Towards LDSL-based multiphysics modeling of semiconductor lasers: Ideals and challenges
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Host
WIAS Berlin
Thursday, 19.11.2020, 16:00 (Online Event)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Konstantinos Mamis, Hellenic Naval Academy, Greece:
Formulating nonlinear Fokker--Planck equations for dynamical systems under coloured noise excitation
more ... Location
Online Event

Host
WIAS Berlin
Thursday, 19.11.2020, 15:00 (Online Event)
TES-Seminar on Energy-based Mathematical Methods and Thermodynamics
Prof. Dr. Christian Wieners, Karlsruher Institut für Technologie:
A discontinuous Galerkin method for phase field approximations of dynamic fracture (online talk)
more ... Location
Online Event

Abstract
We present a new numerical method for dynamic fracture at small strains which is based on a discontinuous Galerkin approximation of a first order formulation for elastic waves and where the fracture is approximated by a phase field driven by a stress based fracture criterion. The staggered algorithm in time combines the implicit midpoint rule for the wave propagation followed by an implicit Euler step for the phase field evolution. Then, driven by a stress based fracture criterion, the material is degradated, and the waves are reflected at the diffusive interface. Then method is evaluated in detail in one dimension, and then we demonstrate in a 2D application the fracture evolution with multiple fractures initiated by reflections. This is joint work with Kerstin Weinberg, Siegen.

Further Informations
Please register on this link: https://mathplus.de/topic-development-lab/tes-winter-2020-21/tes-seminar-on-energy-based-mathematical-methods-and-thermodynamics/

Host
Technische Universität Berlin
WIAS Berlin
Wednesday, 18.11.2020, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Janusz Ginster, Humboldt-Universität zu Berlin:
On the motion of curved dislocations in three dimensions: Simplified linearized elasticity (online talk)
more ... Location
Online Event

Abstract
In this talk we discuss a simplified equilibrium problem for a curved dislocation line in a three-dimensional domain. As the core radius tends to zero, we derive an asymptotic expression to characterize the induced elastic energy. We then obtain the force on the dislocation line as the variation of thi sexpression and identify the highest order terms explicitly. As a main ingredient, we present an explicit asymptotic formula for the induced elastic strain which depends on the curvature of the dislocation line and thus highlights the difference to existing work on straight dislocation lines. Eventually, we present results on the corresponding dynamics. The presented methods area blueprint for the more physical setting of linearized isotropic elasticity. This is joint work with I. Fonseca and S. Wojtowytsch.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 18.11.2020, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Dr. Matthias Löffler, ETH Zürich:
Computationally efficient sparse clustering
more ... Location
Online Event

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 17.11.2020, 13:30 (Online Event)
Seminar Numerische Mathematik
Prof. Peter Berg, University of Alberta, Kanada:
From microscale to macroscale properties of polymer electrolyte membranes: A case for pore network models
more ... Location
Online Event

Abstract
We present the first attempt at describing the flow of water and protons through polymer electrolyte membranes (PEM) by use of an elastic pore network model. The main feature of our approach lies in the interplay between fluxes and pore structures, determined by randomized pore properties, the elasticity of the pores, and the liquid pressure distribution across the network.
Closed-form solutions of the Poisson-Nernst-Planck-Stokes equations along each bond (pore) are employed and coupled to a swelling model for the pores which are embedded in the elastic PEM backbone. All parameter values are taken from the literature, leaving little room for the fitting of model results to literature values.
The resulting nonlinear problem is solved computationally in an efficient manner. More importantly, computed PEM properties at different operating conditions, such as the specific conductivity and the electro-osmotic drag coefficient, compare favourably to values in the literature. In addition, the analysis reveals insights into the nonlinear couplings between transport processes and the structure of the elastic domain which motivates studies of other elastic, nanofluidic systems.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 11.11.2020, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Tomáš Roubíček, Charles University Prague, Czech Republic:
The old Stefan problem in a new mechanical context (online talk)
more ... Location
Online Event

Abstract
The Stefan problem historically describes melting of ice or freezing (solidification) of water as a mere heat-transfer problem with a latent heat. This solid-liquid phase transition however naturally occurs in a mechanical context: melted liquid can flow while frozen solid exhibits some elasticity or some visco-elasticity and even may undergo some inelastic processes as fracture. This needs also to cope with the fluid-solid (so-called fluid-structure) interaction and calls for a model in Eulerian description. Thermomechanical consistency is pursued, as well as a mathematical analysis outlined. The concepts of semi-compressible fluids, viscoelastic solids in Jeffreys' rheology, phase-field fracture, and nonsimple materials (known also as multipolar fluids) will be employed. Some enhancements of this basic thermomechanical scenario will be mentioned, too.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 11.11.2020, 11:30 (WIAS-405-406)
Seminar Interacting Random Systems
Stein Andreas Bethuelsen:
Invariance principle for random walks on dynamically averaging random conductances
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
We prove an invariance principle for continuous-time random walks in a dynamically averaging environment on Z. In the beginning, the conductances may fluctuate substantially, but we assume that as time proceeds, the fluctuations decrease according to a typical diffusive scaling and eventually approach constant unit conductances. The proof relies on a coupling with the standard continuous time simple random walk. Based on joint work with Christian Hirsch (Groningen) and Christian Mönch (Mainz).

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Tuesday, 10.11.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Simon Breneis, WIAS Berlin:
Functions of bounded variation in one and multiple dimensions
more ... Location
Online Event

Abstract
We first recall univariate functions of bounded variation and treat a new result on the Hölder continuities of functions of bounded variation and their variation functions. Then, we discuss possible extensions of the concept of bounded variation to functions of several (though finitely many) variables, compare those generalizations and discuss their uses and shortcomings.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Wednesday, 04.11.2020, 11:30 (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Pierre E. Jacob, Harvard University:
Unbiased Markov chain Monte Carlo methods with couplings
more ... Location
Online Event

Abstract
Various tasks in statistics involve numerical integration, for which Markov chain Monte Carlo (MCMC) methods are state-of-the-art. MCMC methods yield estimators that converge to integrals of interest in the limit of the number of iterations. This iterative asymptotic justification is not ideal; first, it stands at odds with current trends in computing hardware, with increasingly parallel architectures; secondly, the choice of "burn-in" and of the total number of iterations are dificult. This talk will describe recently proposed estimators that are exactly unbiased for the expectations of interest, while having an almost surely finite computing cost and a finite variance. They can thus be generated independently in parallel and averaged over. We argue that the removal of the "burn-in" bias can be done for many MCMC algorithms of practical interest without inflating the variance by much. The core idea is to use coupled pairs of Markov chains following the pathbreaking work of Peter Glynn & Chan-Han Rhee. The method also provides practical upper bounds on some distances between the marginal distribution of the chain at a finite step and its invariant distribution. This talk will provide an overview of this line of research, joint work with John O'Leary, Yves Atchade and many others.

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 03.11.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Darina Dvinskikh, WIAS Berlin:
Improved complexity bounds in Wasserstein barycenter problem
more ... Location
Online Event

Abstract
We focus on computational aspects of Wasserstein barycenter problem. We provide two algorithms to compute Wasserstein barycenter. The first algorithm, based on mirror prox with some specific norm, meets the complexity of celebrated accelerated iterative Bregman projections (IBP), however, with no limitations unlike (accelerated) IBP, that is numerically unstable when regularization parameter is small. The second algorithm, based on area-convexity and dual extrapolation, improves the previously best-known convergence rates for Wasserstein barycenter problem.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 03.11.2020, 10:00 (Online Event)
Forschungsseminar Mathematische Modelle der Photonik
Lasse Ermoneit, TU Berlin:
Semiconductor passively mode-locked lasers subject to opto-electronic feedback
more ... Location
Online Event

Host
WIAS Berlin
November 2 – 4, 2020 (WIAS-405-406)
Workshop/Konferenz: Workshop on Stochastic Geometry and Communications
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Host
WIAS Berlin
Thursday, 29.10.2020, 16:00 (Online Event)
Forschungsseminar Mathematische Modelle der Photonik
Emil Cortes André, Technical University of Denmark:
An analysis of collective effects in nanolasers using minimally extended rate equations
more ... Location
Online Event

Host
WIAS Berlin
Tuesday, 27.10.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Dr. Carsten Hartmann, Brandenburgische Technische Universität Cottbus-Senftenberg:
Convergence to equilibrium in underdamped Langevin dynamics: Fluctuation-dissipation relations, entropy production and optimal control
more ... Location
Online Event

Abstract
We study the convergence to equilibrium of an underdamped Langevin equation that is controlled by a linear feedback force. Specifically, we are interested in sampling the possibly multimodal invariant probability distribution of a Langevin system at small noise (or low temperature) that is characterised by metastability and slow convergence to equilibrium. We follow an approach proposed by Pavon and co-workers [1] and consider a Langevin equation that is simulated at a high temperature, with the control playing the role of a friction that balances the additional noise so as to restore the original invariant measure at a lower temperature. We discuss different limits as the temperature ratio goes to infinity and prove convergence to a limit dynamics. It turns out that, depending on whether the lower (“target”) or the higher (“simulation”) temperature is fixed, the controlled dynamics converges either to the overdamped Langevin equation or to a deterministic gradient flow. This implies that (a) the ergodic limit and the large temperature separation limit do not commute in general, and that (b) it is not possible to accelerate the speed of convergence to the ergodic limit by making the temperature separation larger and larger. We discuss the implications of these observation from the perspective of stochastic optimisation algorithms and enhanced sampling schemes in molecular dynamics. This is joint work with Tobias Breiten, Lara Neureither and Upanshu Sharma [2]. [1] Y. Chen, T.T. Georgiou and M. Pavon. Fast cooling for a system of stochastic oscillators. J. Math. Phys. 56:113302, 2015. [2] T. Breiten, C. Hartmann, L. Neureither and U. Sharma. Stochastic gradient descent and fast relaxation to thermodynamic equilibrium: a stochastic control approach. Preprint, 2020.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Thursday, 22.10.2020, 16:00 (Online Event)
Forschungsseminar Mathematische Modelle der Photonik
Thorsten Svend Rasmussen, Technical University of Denmark:
Multi-purpose modelling of semiconductor Fano lasers: Switching, feedback, and non-linear dynamics
more ... Location
Online Event

Host
WIAS Berlin
Wednesday, 21.10.2020, 11:30 (WIAS-405-406)
Seminar Interacting Random Systems
Michiel Renger:
Variational structures beyond gradient flows: a macroscopic-fluctuation-theory perspective joint work with Rob Patterson (WIAS) and Upanshu Sharma (FU Berlin)
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
Variational formulations in physics are often related to probability via large deviations. Although the idea goes back to Onsager, modern work shows that the large deviations of reversible Markov processes are related to gradient flows. The challenge of non-equilibrium thermodynamics is now to uncover meaningful variational structures for irreversible Markov processes. In that case one can not rule out the occurrence of non-equilibrium steady states, which are best studied by taken particle fluxes into account; this is the approach of “macroscopic fluctuation theory” (MFT). Most literature on MFT study quadratic rate functionals exclusively. These come with a natural concept of orthogonality that allows the decomposition of the flux rate functional into reversible and irreversible parts. We generalise beyond the quadratic case, which is for example relevant for chemical reactions, biological fitness models and epidemiology. As a byproduct we derive a connection to so-called “FIR” inequalities, and we find that the gap is quantified by a large-deviation rate functional of a rather curious process that should be considered `purely irreversible'. Disclaimer: this talk will hardly be about probability, but it will be about analysis of large-deviation rate functionals of Markov processes.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Thursday, 08.10.2020, 14:00 (Online Event)
Seminar Numerische Mathematik
Dr. Yiannis Hadjimichael, WIAS Berlin:
Numeral approximations for spatial-dependent SIR models
more ... Location
Online Event

Abstract
One of the first models used to describe epidemic propagation phenomena is the SIR model, introduced by Kermack and McKendrick in 1927. SIR models split the population into three classes: a group of species susceptible to infection, a compartment of the ill species, and a class of recovered species. In this talk, we consider a generalization of the original model in which the populations' size differs in space. The use of local spatial dependence transforms the SIR model to a system of integro-differential equations. We analyze the uniqueness and stability of the continuous model and suggest suitable numerical approximations that preserve the qualitative properties of the spatial-dependent SIR model. Furthermore, we provide sufficient step-size restrictions under which high order numerical schemes preserve the model's discrete properties (population conservation, positivity, and monotonicity). Finally, computational experiments verify the convergence and accuracy of the numerical methods.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 07.10.2020, 11:30 (WIAS-405-406)
Seminar Interacting Random Systems
Tal Orenshtein:
Aging in the Edwards-Wilkinson and KPZ universality classes
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
Aging is an asymptotic property of non-equilibrium dynamical systems that captures non-trivial relaxation time temporal change; a canonical formulation is expressed in terms of the correlations of the system at two large times with a fixed relation. It was conjectured in Dembo-Deuschel '06 that one-dimensional KPZ models satisfy aging. In line with the progress on the one-time asymptotic behavior of KPZ in the past decade, this challenging problem gained attention in both the physics and the mathematical communities; there has been some experimental evidence for the phenomenon as well as related non-rigorous predictions and partial results. In the talk we shall see that for stationary systems one can use methods that rely solely on the variance asymptotics to achieve aging with an explicit aging function. We shall then derive aging for stationary models in the Edwards-Wilkinson universality class, which is easier to tackle. Moreover, we will demonstrate how to apply the methods to compute a formula for the space-time correlation scaling function in this case. In the remaining part of the talk we shall discuss aging for several stationary models in the KPZ class, including the KPZ fixed point, with the same aging function, matching the stationary KPZ equation prediction in Ferrari-Spohn '16. The talk is based on a recent work with Jean-Dominique Deuschel (TU Berlin) and Gregorio Moreno Flores (PUC Chile).

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Thursday, 01.10.2020, 16:00 (Online Event)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Mindaugas Radziunas, WIAS Berlin:
Simulation of cascaded polarization-coupled systems of broad-area semiconductor lasers
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/89804229771.

Host
WIAS Berlin
Tuesday, 29.09.2020, 10:15 (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Tobias Breiten, Technische Universität Berlin:
Optimal feedback control and minimum energy estimation by value function approximations
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Host
WIAS Berlin
Wednesday, 23.09.2020, 11:30 (WIAS-405-406)
Seminar Interacting Random Systems
Benedikt Jahnel:
The free energy of a grid version of the Bose gas
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
A rigorous derivation of the famous Bose-Einstein condensation in terms of statistical mechanics is widely considered as one of the outstanding open problems in statistical physics.. Starting from a microscopic description of the system as a Gibbs measure of interacting particles in continuous finite volumes, the conjecture is that, in the thermodynamic limit and when the system is forced to have a large particle density, the particles become delocalized and macroscopic components begin to emerge, a condensation phenomenon. One way to make this observation is to consider a reformulation of the problem in terms of a variational formula for the free energy, a strategy which has been successful applied for the non-interacting case. In the talk, I will present a slightly simplified version of the model which allows us to derive the variational description in a way that includes the microscopic and macroscopic parts of the model using large-deviation theory. This is joint work with Wolfgang König.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Wednesday, 09.09.2020, 11:15 (WIAS-HVP-3.13)
Seminar Interacting Random Systems
Heide Langhammer:
Large deviations and the phase transition for the connected components of an inhomogeneous random graph
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Wednesday, 09.09.2020, 10:30 (WIAS-HVP-3.13)
Seminar Interacting Random Systems
Alexandra Quitmann:
Macroscopic cycles in the random loop model
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Thursday, 03.09.2020, 14:00 (Online Event)
Seminar Numerische Mathematik
Prof. Andrea Walther, HU Berlin:
Algorithmic differentiation: Sensitivties, adjoints and nonsmooth optimization
more ... Location
Online Event

Abstract
The provision of exact and consistent derivative information is important for numerous applications arising from optimization purposes as for example optimal control problems. However, even the pure simulation of complex systems may require the computation of derivative information. Implicit integration methods are prominent examples for this case.
The talk will present the technique of algorithmic (or automatic) differentiation (AD) to compute exact derivative information for function evaluations given as computer programs. This includes a short overview of the history of AD and a description of the main variants of AD, namely the forward mode to compute sensitivities as well as the reverse mode for the provision of adjoints. A discussion of complexity estimates follows yielding the important cheap gradient result. Subsequently, I will sketch briefly the usage of our AD tool ADOL-C. Finally, an extension of AD to compute generalized derivatives for nonsmooth optimization is discussed.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Tuesday, 01.09.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Franziska Bielert, Humboldt-Universität zu Berlin:
Estimation of quadratic functionals in a non-parametric regression model and gaussian white noise model
more ... Location
Online Event

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Thursday, 23.07.2020, 14:00 (Online Event)
Seminar Numerische Mathematik
Dr. Zahra Lakdawala, WIAS Berlin:
Supervised and unsupervised learning frameworks for fluid mechanics
more ... Location
Online Event

Abstract
This talk presents an overview of current developments and the future of deep learning for fluid mechanics. The fundamental methodologies for supervised and unsupervised learning will be outlined and the use for understanding, modeling, optimizing and controlling fluid flows will be discussed. For example, how does one incorporate the Navier--Stokes equations or Darcy equations into a deep learning algorithm using neural networks and infer velocity and pressure fields? A framework will be presented where these networks can be integrated within any flow simulation algorithm. We will focus on understanding what a neural network is, how is it trained and how to solve forward and inverse problems where unknown solutions are approximated by a neural network (or a Gaussian process).

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Tuesday, 14.07.2020, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Rico Berner, TU Berlin:
From coherence to incoherence: Stability islands in adaptive neuronal networks
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/83552703765.

Host
Freie Universität Berlin
WIAS Berlin
Technische Universität Berlin
Tuesday, 07.07.2020, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Maria Barbarossa, Frankfurt Institute of Advanced Studies:
Modeling and simulating the early dynamics of COVID-19 in Germany
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/83552703765.

Host
Freie Universität Berlin
WIAS Berlin
Technische Universität Berlin
Tuesday, 07.07.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Paul Hager, Technische Universität Berlin:
Reinforced optimal control
more ... Location
Online Event

Abstract
We extend the reinforced regression methodology from optimal stopping problems to a general class of optimal control problems. The prevailing idea is the reinforcement of the set of basis functions based on previous regressions in the backwards induction steps. Moreover, we propose an efficient modification of the algorithm, which overcomes cost limitations for problems with a high number of possible exercise dates and a high number of possible control states.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 30.06.2020, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Nicolas Perkowski, FU Berlin:
Infinite regularization by noise
more ... Location
Online Event

Abstract
It is a very classical yet still surprising result that noise can have a regularizing effect on differential equations. For example, adding a Brownian motion to an ODE with bounded and measurable vector field leads to a well posed equation with Lipschitz continuous flow. While the equation without noise may have none or many solutions. Classical proofs of such results are based on stochastic analysis and on the close link between the Brownian motion and the heat equation. In that derivation it is not obvious which property of the noise gives the regularization. A more recent approach by Catellier and Gubinelli leads to pathwise conditions under which regularization occurs. I will present a simplified version of their approach and use it to construct (very irregular) paths which are infinitely regularizing: after adding them to an ODE we have a unique solution and an infinitely smooth flow - even if the vector field is only a tempered distribution. This is joint work with Fabian Harang.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/83552703765.

Host
Freie Universität Berlin
Technische Universität Berlin
WIAS Berlin
Thursday, 25.06.2020, 14:00 (Online Event)
Seminar Numerische Mathematik
Holger Stephan, WIAS Berlin:
An overview of Lyapunov functions for linear continuous and discrete problems including some new results
more ... Location
Online Event

Abstract
Lyapunov functions are useful for studying the time behaviour of the solution of evolutionary equations. The classical example is the L_2 norm, which decays on the solution of a diffusion equation with Neumann boundary conditions. The corresponding operator is an operator in divergent form, which forms a symmetric positive definite Dirichlet form. This concept can be generalized to arbitrary linear evolution equations with Markov generators. These equations are exactly those which preserve the type of physical quantities describing them: Intensive quantities (Banach space of continuous functions) are averaged, extensive quantities (Banach space of Radon measures) conserve their mass and sign. It turns out that for Markov generators with real spectrum, there is always a Hilbert space whose norm is a Lyapunov function and whose corresponding Dirichlet form is symmetric and positive. This generalizes the concept of detailed balance. An analogous result applies to the normality of operators with complex spectrum. This is a joint result with Artur Stephan from RG1. Furthermore, an infinite sequence of positive Lyapunov functions -- like energy, dissipation rate and so on -- can be defined recursively. This can be understood as a generalization of the well-known Bakry-Emery theory for diffusion type equations. In the lecture both the finite dimensional discrete case and the infinite dimensional continuous case are considered. Depending on which case is better suited for understanding the nature of the problem, we switch between them in an appropriate manner.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 24.06.2020, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Thomas Eiter, WIAS Berlin:
Spatially asymptotic structure of time-periodic Navier--Stokes flows (online talk)
more ... Location
Online Event

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 24.06.2020, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Dr. Ingo Steinwart, Universität Stuttgart:
Some thoughts and questions towards a statistical understanding of DNNs
more ... Location
Online Event

Abstract
So far, our statistical understanding of the learning mechanisms of deep neural networks (DNNs) is rather limited. Part of the reasons for this lack of understanding is the fact that in many cases the tools of classical statistical learning theory can no longer be applied. In this talk, I will present some thoughts and possible questions that may be relevant for a successful end-to-end analysis of DNNs. In particular, I will discuss the role of initialization, over-parameterization, global minima, and optimization procedures.

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 23.06.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Sara Mazzonetto, Universität Potsdam:
Threshold Ornstein-Uhlenbeck model: Parameters estimation
more ... Location
Online Event

Abstract
A threshold Ornstein-Uhlenbeck is a continuous-time threshold autoregressive process. It follows the Ornstein-Uhlenbeck dynamics when above or below a fixed value, yet at this threshold its coefficients can be discontinuous. We discuss (quasi)-maximum likelihood estimation of the drift parameters, both assuming continuous and discrete time observations. In the ergodic case, we derive consistency and speed of convergence in long time and high frequency of these estimators. Based on these results, we develop a test for the presence of a threshold in the dynamics. Finally, we apply these statistical tools to the short term US interest rates. The talk is based on a joint work with Paolo Pigato.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Wednesday, 17.06.2020, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Dr. François Bachoc, Université Paul Sabatier, Toulouse:
Valid confidence intervals post-model-selection
more ... Location
Online Event

Abstract
In this talk, I will first introduce the post-model-selection inference setting, that has recently been subject to intensive investigation. In the case of Gaussian linear regression, I will review the post-model-selection confidence intervals suggested by Berk et al (2013). These intervals are meant to cover model-dependent regression coefficients, that depend on the selected set of variables. I will present some personal contributions on an adaptation of these confidence intervals to the case where the targets of inference are linear predictors. Then, I will present an extension of these confidence intervals to non-Gaussian and non-linear settings. The suggested more general intervals will be supported by asymptotic results and numerical comparisons with other intervals recently suggested in the literature.

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 16.06.2020, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Sindre W. Haugland, TU München:
A hierarchy of symmetries: Coexistence patterns of four (and more) Stuart--Landau oscillators with nonlinear global coupling
more ... Location
Online Event

Abstract
The symmetry of a system of globally coupled identical units is high: For any solution, interchanging the behavior of any two elements still guarantees a solution. A full N! permutations are possible in total, either not changing the current solution or producing a mirror-image equivalent in phase space. Together, these N! permutations form a symmetry group with many subgroups. Thus, a large number of different partially symmetric solutions might in principle exist. Here, we study N=4 Stuart-Landau oscillators with a particularly intriguing form of global coupling. Its solutions indeed exhibit symmetries corresponding to many of the subgroups of the group of all permutations. Moreover, the transitions between these solutions, together with additional bifurcations more subtly influcencing the symmetry, form an intriguing interconnected hierarchy of differentiated oscillator dynamics. We will also take a brief look at how this hierarchy develops if the ensemble size is scaled up.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/83893529850.

Host
Freie Universität Berlin
Technische Universität Berlin
WIAS Berlin
Tuesday, 16.06.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Valeriy Avanesov, WIAS Berlin:
Data-driven confidence bands for distributed nonparametric regression
more ... Location
Online Event

Abstract
Gaussian Process Regression and Kernel Ridge Regression are popular nonparametric regression approaches. Unfortunately, they suffer from high computational complexity rendering them inapplicable to the modern massive datasets. To that end a number of approximations have been suggested, some of them allowing for a distributed implementation. One of them is the divide and conquer approach, splitting the data into a number of partitions, obtaining the local estimates and finally averaging them. In this paper we suggest a novel computationally efficient fully data-driven algorithm, quantifying uncertainty of this method, yielding frequentist L_2- confidence bands. We rigorously demonstrate validity of the algorithm. Another contribution of the paper is a minimax-optimal high-probability bound for the averaged estimator, complementing and generalizing the known risk bounds.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Wednesday, 10.06.2020, 15:00 (Online Event)
Forschungsseminar Mathematische Statistik
Dr. Jonathan Niles-Weed, New York University:
Minimax estimation of smooth densities in Wasserstein distance
more ... Location
Online Event

Abstract
We study nonparametric density estimation problems where error is measured in the Wasserstein distance, a metric on probability distributions popular in many areas of statistics and machine learning. We give the first minimax-optimal rates for this problem for general Wasserstein distances, and show that, unlike classical nonparametric density estimation, these rates depend on whether the densities in question are bounded below. Motivated by variational problems involving the Wasserstein distance, we also show how to construct discretely supported measures, suitable for computational purposes, which achieve the minimax rates. Our main technical tool is an inequality giving a nearly tight dual characterization of the Wasserstein distances in terms of Besov norms. Joint work with Q. Berthet.

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 09.06.2020, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Peter Koltai, FU Berlin:
Reaction coordinates (order parameters) for metastable systems
more ... Location
Online Event

Abstract
We consider non-deterministic dynamical systems showing complex metastable behavior but no local separation in fast and slow coordinates. We raise the question whether and when such systems exhibit a low-dimensional parametrization supporting their effective statistical dynamics. For answering this question, we aim at finding nonlinear coordinates, called reaction coordinates (or order parameters), such that the projection of the dynamics onto these coordinates preserves the dominant time scales of the dynamics. We show that, based on a specific reducibility property, the existence of good low-dimensional reaction coordinates preserving the dominant time scales is guaranteed. Based on this theoretical framework, we develop and test a novel numerical approach for computing good reaction coordinates. The proposed algorithmic approach is fully local and thus less prone to the curse of dimension with respect to the state space of the dynamics than global methods. Hence, it is a promising method for data-based model reduction of complex dynamical systems such as?but not only?molecular dynamics.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/84070311127.

Host
Freie Universität Berlin
WIAS Berlin
Technische Universität Berlin
Tuesday, 09.06.2020, 10:15 (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Rinaldo Mario Colombo, University of Brescia, Italien:
A hyperbolic-parabolic system to model and control parasites through parasitoids
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Host
WIAS Berlin
June 8 – 10, 2020 (Online Event)
Workshop/Konferenz: 13th Annual ERC Berlin-Oxford Young Researchers Meeting on Applied Stochastic Analysis
more ... Location
Online Event

Host
Technische Universität Berlin
WIAS Berlin
Thursday, 04.06.2020, 11:00 (Online Event)
Seminar Numerische Mathematik
Prof. Volker John, WIAS Berlin:
On the provable convergence order for the kinetic energy of FEMs for the incompressible Navier--Stokes equations
more ... Location
Online Event

Abstract
The kinetic energy of a flow is proportional to the square of the L2(Ω) norm of the velocity. Given a sufficient regular velocity field u and a velocity finite element space with polynomials of degree r, then the best approximation error in L2(Ω) is of order r+1. This talk will discuss which order of convergence for the kinetic energy can be proved with robust error estimates, i.e., with estimates where the constant does not depend on inverse powers of the viscosity. To fix ideas, the first part of the talk is devoted to evolutionary scalar convection-diffusion equations. In the second part, results for robust discretizations of the time-dependent incompressible Navier--Stokes equations will be surveyed. This survey covers as well inf-sup stable pairs of finite element spaces as pressure-stabilized discretizations.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 03.06.2020, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Ingrid van Keilegom, KU Leuven:
On a semiparametric estimation method for AFT mixture cure models
more ... Location
Online Event

Abstract
When studying survival data in the presence of right censoring, it often happens that a certain proportion of the individuals under study do not experience the event of interest and are considered as cured. The mixture cure model is one of the common models that take this feature into account. It depends on a model for the conditional probability of being cured (called the incidence) and a model for the conditional survival function of the uncured individuals (called the latency). This work considers a logistic model for the incidence and a semiparametric accelerated failure time model for the latency part. The estimation of this model is obtained via the maximization of the semiparametric likelihood, in which the unknown error density is replaced by a kernel estimator based on the Kaplan-Meier estimator of the error distribution. Asymptotic theory for consistency and asymptotic normality of the parameter estimators is provided. Moreover, the proposed estimation method is compared with a method proposed by Lu (2010), which uses a kernel approach based on the EM algorithm to estimate the model parameters. Finally, the new method is applied to data coming from a cancer clinical trial.

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 02.06.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Sebastian Riedel, WIAS Berlin:
Runge-Kutta methods for rough differential equations
more ... Location
Online Event

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Thursday, 28.05.2020, 14:00 (Online Event)
Seminar Numerische Mathematik
Dr. Hang Si, WIAS Berlin:
Detri2, a 2D constrained Delaunay-Voronoi mesh generator
more ... Location
Online Event

Abstract
Detri2 is a C++ program to generate triangular meshes from arbitrary polygonal domains.It uses constrained Delaunay algorithms to conform the input line segments and maintain the locally Delaunay property of the mesh toplogy. It uses Delaunay refinement algorithms to generate meshes with good angle bounds. It can generate adaptive meshes corresponding to input mesh sizing functions. In this talk, I will give a brief tutorial of the basic functionalites of Detri2. I will demonstrate the aglorithms and many of the features of Detri2 using a graphic user interface - Detri2Qt. The source code of Detri2 and Detri2Qt are available at http://www.wias-berlin.de/people/si/detri2.html

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 27.05.2020, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Martin Heida, WIAS Berlin:
Stochastic homogenization on randomly perforated domains (online talk)
more ... Location
Online Event

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 26.05.2020, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Eckehard Schöll, TU Berlin:
Partial synchronization patterns in the brain
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/89795202774.

Host
Freie Universität Berlin
WIAS Berlin
Technische Universität Berlin
Tuesday, 26.05.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexandra Suvorikova, WIAS Berlin:
Analysis of 3D chromatin structure using Bures-Wasserstein distance
more ... Location
Online Event

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Wednesday, 20.05.2020, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Chiara Amorino, Université d'Evry Paris-Saclay, France:
Invariant adaptive density estimation for ergodic SDE with jumps over anisotropic classes
more ... Location
Online Event

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 19.05.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Priv.- Doz. Dr. John G. M. Schoenmakers, WIAS Berlin:
Semi-tractability of optimal stopping problems via a weighted stochastic mesh algorithm
more ... Location
Online Event

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 19.05.2020, 10:15 (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Robert Lasarzik, WIAS Berlin:
Dissipative solutions-or-a short crusade against weak solutions
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Host
WIAS Berlin
Thursday, 14.05.2020, 14:00 (Online Event)
Seminar Numerische Mathematik
Derk Frerichs, WIAS Berlin:
Basic concepts of virtual element methods and a really pressure-robust virtual element method for the Stokes problem
more ... Location
Online Event

Abstract
Virtual element methods (VEM) can be seen as a modern approach to extend finite element methods to polygonal and polyhedral meshes. This talk has two aims. First, basic ideas, concepts and advantages of VEM are presented using the well known Poisson problem. The second part concerns a VEM for the Stokes problem. For the Stokes problem exactly divergence-free VEM fail when it comes to small viscosity parameters or when the continuous pressure is complicated. In this talk a modification of the VEM is presented based on Raviart--Thomas approximations of the testfunctions which renders the method really pressure-robust, i.e. locking free for very small viscosities. The construction is also interesting for hybrid hyigh-order methods on polygonal or polyhedral meshes. Numerical results round up the theoretical findings.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Wednesday, 13.05.2020, 11:00 (Online Event)
Seminar Interacting Random Systems
Sam Baguley, Universität Mannheim:
Perpetual Integrals for stable SDEs
more ... Location
Online Event

Abstract
The theory of one-dimensional stochastic differential equations driven by Brownian motion is classical and completely understood for many several decades. For stochastic differential equations with jumps the picture is different, and even the most basic questions are only partially understood. We study the question of existence and uniqueness of weak solutions to $$dZ_t=sigma(Z_t-)dint X_t$$ driven by $$alpha$$-stable Lévy processes.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/86368292467.

Host
WIAS Berlin
Tuesday, 12.05.2020, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Kostya Blyuss, University of Sussex, England:
Mathematical models of epidemics: Insight for control of COVID-19
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/89675974271.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 12.05.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. César A. Uribe, Massachusetts Institute of Technology:
Distributed inference for cooperative learning
more ... Location
Online Event

Abstract
We study the problem of cooperative inference where a group of agents interact over a network and seek to estimate a joint parameter that best explains a set of observations. Agents do not know the network topology or the observations of other agents. We explore a variational interpretation of the Bayesian posterior density, and its relation to the stochastic mirror descent algorithm, to propose a new distributed learning algorithm. We show that, under appropriate assumptions, the beliefs generated by the proposed algorithm concentrate around the true parameter exponentially fast. We provide explicit non-asymptotic bounds for the convergence rate. Moreover, we develop explicit and computationally efficient algorithms for observation models belonging to exponential families.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 12.05.2020, 10:15 (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Robert Gruhlke, WIAS Berlin:
Domain decomposition for random partial differential equations
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Host
WIAS Berlin
Thursday, 07.05.2020, 14:00 (Online Event)
Seminar Numerische Mathematik
Dilara Abdel, WIAS Berlin:
Comparison of modified Scharfetter-Gummel schemes for generalized drift-diffusion systems
more ... Location
Online Event

Abstract
The standard model for the description of semi-classical transport of free electrons and holes in a self-consistent electric field in a semiconductor device is the van Roosbroeck system of equations. For the discretization of this system Voronoi finite volume schemes based on different choices of flux approximations are considered. The classical Scharfetter--Gummel scheme yields a thermodynamically consistent numerical flux, but cannot be used for general charge carrier statistics. This talk aims to give an impression of the comparison and analysis of modified Scharfetter--Gummel schemes to simulate generalized drift-diffusion systems.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Thursday, 07.05.2020, 10:00 (Online Event)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Matthias Wolfrum, WIAS Berlin:
Temporal dissipative solitons in delay-differential equations
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/81751744679.

Host
WIAS Berlin
Wednesday, 06.05.2020, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Boris Zaltzman, Ben Gurion University of the Negev, Israel:
Equilibrium electro-convective instability in electrodeposition with Butler--Volmer kinetics (online talk)
more ... Location
Online Event

Abstract
Isaak Rubinstein and Boris Zaltzman. In this presentation we report that equilibrium electro-convective instability is possible in concentration polarization in the course of cathodic electrodeposition. The cathode is modelled as an ideally perm-selective interface with infinite lateral conductivity. The cation transfer across the cathode/solution interface is assumed to obey the Butler--Volmer kinetics with parameters typical of copper deposition. It is shown that deviation from the local reaction equilibrium due to the final deposition reaction rate renders possible the equilibrium electro-convective instability with a critical wavelength on the scale of diffusion layer width. This scaling may be recognized as a characteristic signature of equilibrium instability as opposed to the non-equilibrium one, related to the extended space charge. This latter instability, owning to its shortwave character, is characterized by origination of critical small vortices with a wavelength considerably shorter that the width of the diffusion layer. Interaction of these small scale vortices yields their fusion through which they evolve until reach a size comparable to the width of the diffusion layer.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 06.05.2020, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Dr. Julia Schaumburg, VU Amsterdam, Netherlands:
Dynamic clustering of multivariate panel data
more ... Location
Online Event

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 05.05.2020, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Prof. Carlos Rocha, Instituto Superior Tecnico, Lisboa, Portugal:
Meanders, zero numbers and the cell structure of Sturm global attractors
more ... Location
Online Event

Abstract
We overview recent results on the geometric and combinatorial characterization of global attractors of semiflows generated by scalar semilinear partial parabolic differential equations under Neumann boundary conditions. This is joint work with B. Fiedler.

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/83594452457.

Host
GAMM (AG UQ)
WIAS Berlin
Tuesday, 05.05.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Oleg Butkovsky, WIAS Berlin:
Stochastic sewing with controls and skew fractional Brownian motion
more ... Location
Online Event

Abstract
I will explain how sewing with controls developed by Peter Friz & Huilin Zhang can be extended to the stochastic setup and show how this can be used to establish existence of skew fractional brownian motion and related SDEs with iregular drift.

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 05.05.2020, 14:00 (Online Event)
Seminar Numerische Mathematik
Dr. Christian Merdon, WIAS Berlin:
A novel gradient-robust, well-balanced discretisation for the compressible Stokes problem
more ... Location
Online Event

Abstract
This talk suggests a novel well-balanced discretisation of the stationary compressible isothermal Stokes problem based on the concept of gradient-robustness. Gradient-robustness means that arbitrary gradient fields in the momentum balance are well-balanced by the discrete pressure gradient - if there is enough mass in the system to compensate the force. The scheme is provably convergent and asymptotic-preserving in the sense that it degenerates for low Mach numbers to a recent inf-sup stable and pressure-robust modified Bernardi--Raugel finite element method for the incompressible Stokes equations. All properties are demonstrated in numerical examples.

Further Informations
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Host
WIAS Berlin
Tuesday, 05.05.2020, 10:15 (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Andreas Rathsfeld, WIAS Berlin:
Scatterometry and rough surfaces
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Host
WIAS Berlin
Wednesday, 29.04.2020, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Boris Zaltzman, Ben-Gurion University of the Negev, Israel:
Orientation effects in hydrodynamic instability in concentration polarization (online talk)
more ... Location
Online Event

Abstract
Pramoda Kumar, Isaak Rubinstein and Boris Zaltzman. One dimensional steady state passage of the electric current from an aqueous electrolyte solution into a charge selective (perm-selective) solid, such as a metal electrode or ion exchange membrane, is hydro-dynamically unstable, [1]-[3]. For a long time this instability has been attributed to electro-convection related to the extended space charge which forms at the outer edge of the electric double layer at the limiting current, [4]-[10]. For the perfectly perm-selective solid with infinite conductivity this mechanism is the only possible one. Recently, it has been shown that for a non-perfectly perm-selective membrane, or for a perfectly perm-selective membrane with finite conductivity (commercial ion-exchange membranes are practically perfectly perm-selective), additional electro-convective instability mechanisms non-related to the extended space charge are possible, [11]-[13]. All of the afore mentioned studies focused on electro-convective instability mechanisms because it was recognized the gravitational mechanism could not yield instability for most practical situations (sub-millimeter electrolyte systems with concentration below one centimolar and without a forced stirring). In such systems, the passage of electric current results in formation of concentration variations near the perm-selective solid known as concentration polarization (CP). The expression of CP is a characteristic voltage current dependence with a current saturation at the limiting value (limiting current, corresponding to a nearly vanishing interface electrolyte concentration) followed by a current increase with voltage known as the over-limiting conductance regime. For some systems, transition to the over-limiting conductance is accompanied by the appearance of excessive electric noise. Commonly, over-limiting conductance sets on due to the mechanical stirring of the interface diffusion layer resulting from a hydrodynamic electroconvective instability of quiescent electric conduction. In relation to possible gravitational mechanisms, the CP-induced density stratification results both from direct concentration variation and its related non-uniform Joule heating.The temperature variations related to the latter usually do not exceed one degree Kelvin. As a result, for gravitationally unstable orientation with the depleted interface at the bottom, the Rayleigh number related to the thermally induced density stratification is usually smaller than that related to concentration variation, whereas the latter is lower than the Rayleigh--Benard instability threshold. For this reason, electro-convective mechanisms have been invoked to explain the instability, and no systematic study of the Joule heating effects in CP has been undertaken until recently (not long ago, the effect of density stratification related to concentration variation upon the electroconvective instability in CP has been addressed in Ref. [14]). Lately, this shortcoming has been remedied in the theoretical studies by Demekhin and his group, [15]. They discovered a very interesting possibility of a Joule heating related instability expected to occur for overall stable density stratification (depleted interface on the top). In this presentation we investigate this possibility both experimentally and theoretically. References 1. G. Yossifon and H. C. Chang, Phys. Rev. Lett.101, 254501 (2008). 2. S. M. Rubinstein, G. Manukyan, A. Staicu, I. Rubinstein, B. Zaltzman, R. G. H. Lammertink, F. Mugele, and M. Wessling, Phys. Rev. Lett. 101, 236101 (2008). 3. J.C. de Valenc, R. M.Wagterveld, R. G. H. Lammertink, and P.A.Tsai, Phys.Rev.E 92, 031003(R) (2015). 4. I. Rubinstein and L. Shtilman, J. Chem. Soc. Faraday Trans. II 75, 231 (1979). 5. A.V.Listovnichy, Elektrokhimia 25, 1651 (1989). 6. V. V. Nikonenko, V. I. Zabolotsky, and N. P. Gnusin, Sov. Electrochem. 25, 262 (1989). 7. I. Rubinstein and B. Zaltzman, Phys. Rev. E62, 2238 (2000). 8. I. Rubinstein and B. Zaltzman, Math. Mod. Meth. Appl. Sci. 11, 263 - 300 (2001). 9. B. Zaltzman and I. Rubinstein, J. Fluid Mech. 579, 173 (2007). 10. C.L.Druzgalski, M.B.Andersen,and A. Mani, Phys. Fluids 25, 110804 (2013). 11. I. Rubinstein and B. Zaltzman, Phys. Rev. Lett.114, 114502 (2015). 12. R. Abu-Rjal, I. Rubinstein, and B. Zaltzman, Phys. Rev. Fluids 1, 023601 (2016). 13. I. Rubinstein and B. Zaltzman, Phys.Rev. Fluids 2, 093702 (2017). 14. E. Karatay, M.B.Andersen, M.Wessling, and A. Mani, Phys. Rev. Lett.116, 194501 (2016). 15. E. A. Demekhin, S. Amiroudine, G. S. Ganchenko, and N. Y. Khasmatulina, Phys. Rev. E91, 063006 (2015).

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 29.04.2020, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Dr. Thorsten Dickhaus, Universität Bremen:
How many null hypotheses are false?
more ... Location
Online Event

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 28.04.2020, 15:15 (Online Event)
Oberseminar Nonlinear Dynamics
Dr. Markus Kantner, WIAS Berlin:
Beyond just ,,flattening the curve”: Optimal control of epidemics with purely non-pharmaceutical interventions
more ... Location
Online Event

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 28.04.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Darina Dvinskikh, WIAS Berlin:
SA vs SAA for population Wasserstein barycenter calculation
more ... Location
Online Event

Further Informations
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 28.04.2020, 10:15 (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
David Sommer, WIAS Berlin:
A variable transformation for the Hamilton Jacobi Bellman equation
more ... Location
Online Event

Further Informations
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Host
WIAS Berlin
Wednesday, 22.04.2020, 15:15 (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Boris Zaltzman, Ben Gurion University of the Negev, Israel:
Artifact of “Breakthrough” osmosis or what may be wrong with local Spiegler--Kedem--Katchalsky equations with constant coefficients (online talk)
more ... Location
Online Event

Abstract
Isaak Rubinstein and Boris Zaltzman. Osmosis - solution (solvent) flow through non-perfectly (perfectly) semipermeable membranes - is a fundamental classical phenomenon of major practical importance. One of its potentially useful technological applications is the Pressure Retarded Osmosis employed for energy harvesting from salinity variations. In this process the flow resulting from the osmotic pressure drop between fresh and saline water is used to drive a turbine. Unfortunately, at the current stage, in spite of its extreme simplicity and conceptual beauty, this process does not appear to be practically viable due to insufficient power efficiency. This assessment could be radically changed by operating pressure retarded osmosis the recently theoretically predicted “Breakthrough” operation mode. In this mode, the solute concentration at the interface between the porous support and the dense selective barrier layer of a non-perfect (“leaky”) asymmetric membrane employed in process decreases with the increase of draw concentration, and, thus, the impeding effect of internal concentration polarization is eliminated. The existence of this mode was predicted based on the analysis of the system of classical local Spiegler--Kedem--Katchalsky equations of membrane transport in the barrier layer employing three constant coefficients: solute permeability, solute reflection coefficient and hydraulic permeability. In a still more recent study the physical feasibility of “Breakthrough” mode was contested and doubt was casted upon the suitability of Spiegler--Kedem--Katchalsky equations with constant coefficients for modeling of Pressure Retarded Osmosis. We re-derive the Spiegler--Kedem--Katchalsky equations based on a simple capillary friction model of membrane transport in the dense barrier layer and identify the problem with the constant coefficients' assumption resulting in the occurrence of “Breakthrough” mode. Our derivation recovers the Spiegler--Kedem--Katchalsky equations in the dilute solution limit, albeit with hydraulic permeability dependent on the local solute concentration in the barrier layer. Taking into account this dependence, necessary for preserving the detailed force balance in the barrier layer, eliminates the existence of the “Breakthrough” mode.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 22.04.2020, 10:00 (Online Event)
Forschungsseminar Mathematische Statistik
Gabriel Peyré, Ecole Normale Supérieure Paris:
Scaling optimal transport for high dimensional learning
more ... Location
Online Event

Further Informations
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 21.04.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Nikita Zhivotovskiy, Google Research:
Fast classification rates in online and statistical learning with abstention
more ... Location
Online Event

Further Informations
Dieser Vortrag findet bei Zoom statt: http://zoom.us/j/492088715

Host
WIAS Berlin
Tuesday, 21.04.2020, 14:00 (Online Event)
Seminar Materialmodellierung/ Seminar Numerische Mathematik
Dr. Alfonso Caiazzo, WIAS Berlin:
Modeling of biological flows and tissues
more ... Location
Online Event

Abstract
The talk shows results of ongoing research in RG 3 related to modeling of blood flows and biological tissues. After a brief introduction on medical imaging techniques - especially magnetic resonance elastography - the first part of the talk wil discuss multiscale models for the simulation of vascularized tissues, using immersed methods. As next, we will introduce the equations and the challenges in computational hemodynamics, recent results and recently started collaborations.

Further Informations
For zoom login details please contact Dirk Peschka http://www.wias-berlin.de/ peschka/?lang=1

Host
WIAS Berlin
Tuesday, 14.04.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexandra Suvorikova, WIAS Berlin:
On some general pricniples behind the results on the bootstrap validity in Bures-Wasserstein space
more ... Location
Online Event

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Tuesday, 14.04.2020, 11:00 (Online Event)
Seminar Interacting Random Systems
Prof.Dr. Wolfgang König, WIAS Berlin:
Condensation in the interacting Bose gas in the semi-classical mean-field setting
more ... Location
Online Event

Further Informations
online seminar

Host
WIAS Berlin
Tuesday, 07.04.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Dr. Vladimir Spokoiny, WIAS Berlin:
Bayesian inference for nonlinear inverse problems
more ... Location
Online Event

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Tuesday, 07.04.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Dr. Vladimir Spokoiny, WIAS Berlin:
Bayesian inference for nonlinear inverse problems
more ... Location
Online Event

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Tuesday, 31.03.2020, 15:00 (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Karsten Tabelow, WIAS Berlin:
Model-based geometry reconstruction (MGBR) from TEM
more ... Location
Online Event

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Thursday, 12.03.2020, 13:00 (WIAS-406)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Olivier Huber, HU Berlin:
Topics in gas transport: Nash equilibrium and constrained exact boundary controllability
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We present two results related to the transport of gas: the existence of a solution to a Generalized Nash Equilibrium Problem (GNEP) arising from the modeling of the gas market as an oligopoly, that is only the producers are players, and the consumers just react to the quantity of gas available. In a second part, the constrained exact boundary controllability of a semilinear hyperbolic PDE is investigated. The existence of an absolutely continuous solution and boundary control will be shown, under appropriate assumptions.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
March 12 – 13, 2020 (WIAS-ESH)
Workshop/Konferenz: International Workshop: Uncertainty and Fluctuations in Thermodynamics
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Thursday, 05.03.2020, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Alexander Linke, WIAS Berlin:
Gradient-robustness: A new concept assuring accurate spatial discretizations for vector-valued PDEs
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Vector-valued PDEs like the incompressible Navier--Stokes equations (in primitive variables velocity and pressure) describe the time evolution of a vector-valued quantity like the momentum density. For vector-valued PDEs it is quite natural to derive formally on the continuous level a derived time evolution of the vorticity and the divergence. Accordingly, any space discretization for a vector-valued PDE (implicitly) delivers a discrete vorticity and discrete divergence equation.
While the celebrated inf-sup stability will be shown to assure an accurate discrete divergence equation, the talk will actually focus on the question, which structural properties allow for an implicitly defined accurate discrete vorticity equation. The key observation is that the L^2-orthogonality of divergence-free vector fields and gradient fields is the weak equivalent to the vector calculus identity $nabla times nabla psi = mathbf0$ for arbitrary smooth scalar fields $psi$.
In the context of the incompressible Navier-Stokes equations, the concept of pressure-robustness was introduced in 2016, in order to discriminate between space discretizations with accurate and inaccurate discrete vorticity equations. H(div)-conforming finite element spaces have been found as an important means to realize the L^2-orthogonality between discretely divergence-free vector fields and (arbitrary) gradient fields. Further, it is shown that spatial discretizations that are not pressure-robust may suffer from i) a degradation of the (preasymptotic) convergence rate, and ii) large, parameter-dependent constants in error estimates. Typical flows that benefit from pressure-robustness are quasi-hydrostatic flows, quasi-geostrophic flows and vortex-dominated high Reynolds number flows.
Last but not least, the talk shows how to extend the concept of pressure-robustness to more general vector-valued PDEs, leading to the concept of gradient-robustness. Gradient robustness assures that dominant and extreme gradient fields in a vector-valued PDE will not lead to inaccuracies in the discretization. The talk will show examples from linear elasticity and compressible (Navier--)Stokes flows at low Mach numbers and in stratified flows. Thus, connections to well-balanced schemes and WENO schemes will be drawn.

Host
WIAS Berlin
Thursday, 27.02.2020, 15:00 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Anton Stephan, Institut für Physik der Atmosphäre:
Modeling Navier--Stokes equations for high Reynolds number flows and complex geometries in aerodynamic applications
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The flight of an aircraft through a turbulent environment presents a difficult numerical problem. Though it can be easily described by the Navier-Stokes equations, it is impossible to perform a direct numerical simulation resolving all physically relevant scales. For several decades, numerous different models have been developed which perform very well for very specific problems. However, since the aircraft flight includes complex geometries as well as large spatial dimensions these models fail to capture all important features of the problem.
The vision of establishing a simulation system for virtual flight in a realistic environment is addressed by the coupling of two separate flow solvers in a bi-directional manner. This enables for example the simulation of a flight through gusts, trailing vortices and clear air turbulence, including the effects on the aircraft and the roll-up of trailing vortices and their further development until final decay. A compressible Reynolds-Averaged Navier-Stokes (RANS) solver resolves the near-field around an aircraft model. An incompressible Large-Eddy Simulation (LES) solver is used to model the atmosphere around the aircraft with its wake footprint in the LES domain. This hybrid simulation setup is used to investigate complex physical problems occurring in different flight scenarios.

Host
WIAS Berlin
Thursday, 27.02.2020, 15:00 (WIAS-HVP-3.13)
Seminar Laserdynamik
Maksymilian-Gregor Siwik, TU Berlin:
Calculation of Andronov--Hopf-Bifurcations of a PDE using the continuation and bifurcation software Auto-07P
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Host
WIAS Berlin
Thursday, 27.02.2020, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Philip Lederer, TU Wien, Österreich:
Divergence-free tangential finite element methods for incompressible flows on surfaces
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this work we consider the numerical solution of incompressible flows on twodimensional manifolds. Whereas the compatibility demands of the velocity and the pressure spaces are known from the flat case one further has to deal with the approximation of a velocity field that lies only in the tangential space of the given geometry. Abandoning H1-conformity allows us to construct finite elements which are -- due to an application of the Piola transformation -- exactly tangential. To reintroduce continuity (in a weak sense) we make use of (hybrid) discontinuous Galerkin techniques. To further improve this approach, H(div Γ)-conforming finite elements can be used to obtain exactly divergence-free velocity solutions. We present several new finite element discretizations. On a number of numerical examples we examine and compare their qualitative properties and accuracy.

Host
WIAS Berlin
Thursday, 27.02.2020, 11:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Axel Kröner, WIAS Berlin:
Optimal control of a semilinear heat equation subject to state and control constraints
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 25.02.2020, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Dr. Håkon Hoel, RWTH Aachen:
Multilevel ensemble Kalman filtering algorithms
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Thursday, 20.02.2020, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Wolfgang Dreyer, WIAS Berlin:
Classical tests of electro-chemistry
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Volume changes, heat of dilution, osmosis, saturation pressure decrease and boiling temperature increase of electrolytes are classical phenomena that can be used to test and adjust parameter of electrochemical modeling. In this lecture we apply the electrolyte model developed at WIAS to these phenomena in order to critically examine the underlying assumptions. The observations require generalizations of the notions of incompressibility and surface tension.

Host
WIAS Berlin
Tuesday, 18.02.2020, 13:30 (WIAS-ESH)
Seminar Numerische Mathematik
Ondřej Pártl, WIAS Berlin:
Mathematical modeling of non-isothermal compositional compressible fluid flow in zeolite bed and above its surface
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
I will present a mathematical model and numerical scheme for the simulation of non-isothermal compositional compressible fluid flow in a heterogeneous porous medium and in the coupled atmospheric boundary layer above the surface of this porous medium, where one of the flowing components adsorbs on the porous matrix. I will discuss the application of this model to the simulation of the hydration of a zeolite bed.
My model is based on the two-domain approach, i.e., the domain in which the flow occurs is divided into the porous medium subdomain and the free flow subdomain. In each subdomain, the flow is described by the corresponding balance equations for mass, momentum and energy. On the interface between the subdomains, coupling conditions are prescribed.
In both subdomains, the spacial discretization of the governing equations is carried out via the finite volume method. For time integration, we use a special time stepping procedure.

Host
WIAS Berlin
Thursday, 13.02.2020, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Gabriel R. Barrenechea, University of Strathclyde, Scotland:
Low-order divergence-free finite element methods
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this talk I will review results on a divergence-free reconstruction of the lowest order pair for the Navier-Stokes equation. More precisely, from a stabilised P1xP0 scheme, a divergence-free velocity field is built as the result of a lift of the pressure jumps, and it is then plugged in the convective term of the momentum equation. This process provides a method that can be proven stable without the need to suppose the mesh refined enough. We first apply this idea to the transient Navier-Stokes equations, where estimates independent of the viscosity are derived. Then, the applicability of this idea is extended to a steady-state generalised Boussinesq system.

Host
WIAS Berlin
Wednesday, 12.02.2020, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Fredi Tröltzsch, Technische Universität Berlin:
Optimal and feedback control of some reaction-diffusion equations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The optimal control of some systems of nonlinear reaction-diffusion equations is considered including the Nagumo and FitzHugh--Nagumo equations. These equations are known to develop traveling wave fronts, spiral waves or scroll rings as solutions. Well-posedness of the system, differentiability of the control-to-state mapping, and optimality conditions of first order are briefly sketched. In particular, the case of sparse optimal control is addressed. Moreover, the optimization of time-delays in local and nonlocal Pyragas type feedback control up to the control by measures is discussed. Various numerical examples illustrate the great diversity of geometrical patterns and their control.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 12.02.2020, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Alexandra Carpentier, Universität Magdeburg:
Adaptive inference and its relations to sequential decision making
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Adaptive inference - namely adaptive estimation and adaptive confidence statements - is particularly important in high of in nite dimensional models in statistics. Indeed whenever the dimension becomes high or infinite, it is important to adapt to the underlying structure of the problem. While adaptive estimation is often possible, it is often the case that adaptive and honest con dence sets do not exist. This is known as the adaptive inference paradox. And this has consequences in sequential decision making. In this talk, I will present some classical results of adaptive inference and discuss how they impact sequential decision making. (based on joint works with Andrea Locatelli, Matthias Loeffler, Olga Klopp and Richard Nickl)

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 11.02.2020, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Peter Szmolyan, Technische Universität Wien, Austria:
Advances in GSPT -- Teaching new tricks to an old dog
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstract
Due to the efforts of many people geometric singular perturbation theory (GSPT) has developed into a very successful branch of applied dynamical systems. GSPT has proven to be very useful in the analysis of an impressive collection of diverse problems from natural sciences, engineering and life sciences. Fenichel theory for normally hyperbolic critical manifolds combined with the blow-up method at non-hyperbolic points is often able to provide remarkably detailed insight into complicated dynamical phenomena, often even in a constructive way. Much of this work has been carried out in the framework of slow-fast systems in standard form, i.e. for systems with an a priori splitting into slow and fast variables. More recently GSPT turned out to be useful for systems for which the slow-fast structures and the resulting applicability of GSPT are somewhat hidden. Problems of this type include singularly perturbed systems in nonstandard form, problems depending singularly on more than one parameter and smooth systems limiting on non-smooth systems as a parameter tends to zero. Often several distinct scalings must be used to cover the dynamics of interest and matching of these different scaling regimes is carried out by the blow-up method.. In this talk I will survey these developments and highlight some ongoing activities.

Host
Freie Universität Berlin
WIAS Berlin
Thursday, 06.02.2020, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Holger Stephan, WIAS Berlin:
A general concept of majorization and a corresponding Robin Hood method
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The Robin Hood method is a numerical method for constructing a double-stochastic matrix that maps a given vector to another vector that is also given. This is possible if the preimage majorizes the image. The majorization of vectors is a special order sometimes called the Lorentz order. A double-stochastic matrix is a special case of a Markov (or stochastic) matrix. In the case of a general Markov matrix, it is not yet clear when such a construction is possible, what kind of condition similar to the majorization must be fulfilled by the given vectors, nor is a numerical method for determining the matrix known (regardless some heuristic iterative versions of the Robin-Hood method).
In the talk, we give a complete solution to all these problems, including a general direct Robin Hood method. It turns out that the construction of a Markov matrix from two given pairs of two vectors is possible if and only if the pairs satisfy a certain order condition. If one interprets the vectors as states of a physical system, then this order is exactly the order known as the natural time order or the second law of thermodynamics.

Host
WIAS Berlin
Wednesday, 05.02.2020, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Riccardo Cristoferi, Heriot-Watt University:
Epitaxial crystal growth with adatoms
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Surface diffusion is one of the most important mechanisms driving crystal growth. When bulk diffusion is much faster than the surface one, the evolution of the profile of the crystal is described by the so called Einstein--Nernst relation. According to this law, the normal velocity of the profile is related to the chemical potential. This evolution equation has a variational flavor, in that it can be obtained as a gradient flow of a suitable free energy. Albeit usually neglected, adatoms (atoms freely diffusing on the surface of the crystal) seem to play a fundamental role in the description of the behaviour of a solid-vapor interfaces. For this reason, some years ago Fried and Gurtin introduced a system of evolution equations to describe such a situation, where adatoms are treated as a separate variable of the problem. Also such system of evolution equations can be seen as the gradient flow of an energy. In this talk a first step in the programme of studying the above mentioned evolution equations from a variational point of view is presented. In particular, the focus is on the static problem in the small mass regime, where the elastic energy is negligible. Ground states, effective energy, and phase filed approximation suitable for numerical purposes are discussed. This latter is embedded in a general framework that allows to treat similar problems for a large class of functionals. The talk is based on works in collaboration with Marco Caroccia (Roma Tor Vergata), and Laurent Dietrich (Lycée Fabert).

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 05.02.2020, 11:30 (WIAS-406)
Seminar Interacting Random Systems
Richard Pymar, Birkbeck University of London:
Mixing times of exclusion processes on regular graphs
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
Place k black particles and n-k white particles on the vertices of an n vertex graph, with one per vertex. Suppose each edge rings at rate 1 independently, and when an edge rings particles at the end-points switch positions. Oliveira conjectured that this "k-particle exclusion process" has mixing time of order at most that of k independent particles. Together with Jonathan Hermon we prove a bound for regular graphs which is in general within a log log n factor from this conjecture when k>n^c and which, in certain cases, verifies the conjecture. As a result we obtain new mixing time bounds for the exclusion process on expanders and the hypercube.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Wednesday, 05.02.2020, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Tim Sullivan, Freie Universität Berlin:
A rigorous theory of conditional mean embeddings
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Conditional mean embeddings (CME) have proven themselves to be a powerful tool in many machine learning applications. They allow the efficient conditioning of probability distributions within the corresponding reproducing kernel Hilbert spaces (RKHSs) by providing a linear-algebraic relation for the kernel mean embeddings of the respective probability distributions. Both centered and uncentered covariance operators have been used to define CMEs in the existing literature. In this talk, we develop a mathematically rigorous theory for both variants, discuss the merits and problems of either, and significantly weaken the conditions for applicability of CMEs. In the course of this, we demonstrate a beautiful connection to Gaussian conditioning in Hilbert spaces.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Monday, 03.02.2020, 14:00 (WIAS-ESH)
INSTITUTSKOLLOQUIUM
Prof. Dr.-Ing. Georg Duda, Charité Berlin, Julius Wolff Institute for Biomechanics and Musculoskeletal Regeneration:
From Mechanical Forces to Tissue Straining - How to Employ Biophysical Cues to Guide Regeneration
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Conceptually, our research aims at understanding endogenous cascades of tissue formation, cytokine signaling and cellular self-organization especially in bone. Mechanical straining and adaptation due to mechanical cues plays a central role in all these tissues. The aim of our work is to understand the mechano-biological cues of regeneration and adaptation and how they can be employed to enable healing even in tissues with impaired regenerative capacity such as muscle, cartilage or tendon. All approaches are motivated by clinical challenges, are based on in vivo patient measurements, employ basic research principles and aim at being translated into daily clinical routine. Examples of translation include innovative concepts for joint replacement procedures, angle stable fixation of implants or cell therapies for immune-modulation to empower tissue regeneration.

Further Informations
Presentation slides

Host
WIAS Berlin
Wednesday, 29.01.2020, 10:00 (WIAS-405-406)
Forschungsseminar Mathematische Statistik
Nadja Klein, Humboldt-Universität zu Berlin:
Bayesian regression copulas
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 28.01.2020, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Martin Brokate, TU München:
Sensitivity results for rate independent evolutions
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstract
Rate independent evolutions are evolutions whose solution operators commute with time transformations. They are inherently nonlinear and nonsmooth. We present some introduction, outlining various mathematical approaches as well as relations to other fields. We then address sensitivity, in particular the question whether the solution operators possess weak differentiability properties.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 28.01.2020, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Anastasia Ivanova, Moscow Institute of Physics and Technology (MIPT), Russische Föderation:
Optimization methods for resource allocation problem
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Further Informations
Forschungsseminar “Mathematische Statistik”

Host
WIAS Berlin
January 28 – 30, 2020 (WIAS-ESH)
Workshop/Konferenz: Workshop on PDE Constrained Optimization under Uncertainty and Mean Field Games
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Wednesday, 22.01.2020, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Joachim Rehberg, WIAS Berlin:
The numerical range of Dirichlet forms
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 22.01.2020, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Jorge Mateu, Universitat Jaume I, Valencia, Spain:
Complex spatial and spatio-temporal point process dependencies: Linear ANOVA-type models, metrics and barycenters and predictive stochastic models of crime
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 21.01.2020, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr Pavel Dvurechensky, WIAS Berlin:
On the complexity of optimal transport problems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Host
WIAS Berlin
Friday, 17.01.2020, 14:00 (WIAS-ESH)
INSTITUTSKOLLOQUIUM
Prof. Stefan Heinrich, Universität Kaiserslautern, Germany:
Quantum computing for numerical problems - algorithms and complexity
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
After a brief introduction into quantum computation we survey results on the speedup quantum algorithms can reach over classical (i.e. non-quantum) deterministic and randomized algorithms. We focus on basic numerical problems including integration, approximation and the solution of PDEs. We compare optimal convergence rates in the quantum setting with those in the deterministic and randomized classical setting. Upper bounds are obtained by suitable algorithms and their analysis, lower bounds by using the framework of information-based complexity theory.

Host
WIAS Berlin
January 17, 2020 (WIAS-ESH)
Workshop/Konferenz: Workshop on the occasion of the retirement of Peter Mathé
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Wednesday, 15.01.2020, 11:30 (WIAS-406)
Seminar Interacting Random Systems
Marcel Fenzl, Universität Zürich, Schweiz:
Asymptotic results for stabilizing geometric statistics
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
Even though global in nature, many geometric statistics like the germ-grain model or the $k$-nearest neighbour model can be realized as a sum of local contributions. This idea can be formalized by using the concept of stabilizing score functions. In this talk, we investigate geometric statistics arising from such stabilizing score functions for randomly chosen input points. More specifically, we analyse the asymptotic behaviour of such a statistic for general point processes with fast decaying correlation functions.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Wednesday, 15.01.2020, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Sven Wang, University of Cambridge, UK:
Convergence rates for penalised least squares estimators in PDE-constrained regression problems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 14.01.2020, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Frits Veerman, University of Heidelberg:
Toll roads and freeways: Defects in bilayer interfaces in the multi-component functionalised Cahn--Hilliard equation
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstract
We study a multi-component extension of the functionalised Cahn-Hilliard equation, which provides a framework for the formation of patterns in fluid systems with multiple amphiphilic molecules. The assumption of a length scale dichotomy between two amphiphilic molecules allows the application of geometric techniques for the analysis of patterns in singularly perturbed reaction-diffusion systems. For a generic two-component system, we show that solutions to the four-dimensional connection problem provide the leading order approximation for solutions to the full eight-dimensional barrier problem, which can be obtained through a perturbative expansion in the layer width. Moreover, we show that a saddle-node bifurcation of bilayer solutions in the four-dimensional connection problem acts as a source of so-called defect solutions, i.e. solutions to the barrier problem that are not also solutions to the connection problem. The analysis combines geometric singular perturbation theory with centre manifold theory in an infinite-dimensional context.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 14.01.2020, 15:00 (WIAS-ESH)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexander Gasnikov, Moscow Institute of Physics and Technology:
An overview of distributed optimization
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Thursday, 09.01.2020, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Lia Strenge, Technische Universität Berlin:
A multilayer, multi-timescale model approach for economic and frequency control in power grids using Julia
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Power systems are subject to fundamental changes due to the increasing infeed of decentralized renewable energy sources and storage. The decentral nature of the new actors in the system requires new concepts for structuring the power grid, and achieving a wide range of control tasks ranging from seconds to days. Here we introduce a multilayer dynamical network model covering a wide range of control time scales. Crucially we combine a decentralized, self-organized low-level control and a smart grid layer of devices that can aggregate information from remote sources. The stability critical task of frequency control is performed by the former, the economic objective of demand matching dispatch by the latter. Having both aspects present in the same model allows us to study the interaction between the layers. Remarkably we find that adding communication in the form of aggregation does not improve the performance in the cases considered. Instead the self-organised state of the system already contains the information required to learn the demand structure in the entire power grid. The model introduced here is highly flexible, and can accommodate a wide range of scenarios relevant to future power grids. We expect that it will be especially useful in the context of low-energy microgrids with distributed generation. All simulations and numerical experiments for control design and analysis with sampling-based methods are performed in Julia 1.1.0. The overall model is implemented as stiff nonlinear ordinary differential equation (ODE) with periodic callbacks for the control actions. The ODE has dimension 4 N, where N is the number of edges of the graph representing the power grid (i.e., N feed-in/load connections). It is planned to use automatic differentiation to learn more about the overall nonlinear model.

Host
WIAS Berlin
Wednesday, 08.01.2020, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Michiel Renger, WIAS Berlin:
Fast reaction limits via Gamma-convergence of the flux rate functional
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We consider a system of linear ODEs on a finite graph with multiple time scales, and revisit the classical problem of deriving effective equations for the evolution. If solutions of the ODEs minimise an action functional, one can study Γ-convergence of that functional, and find the effective equations as the minimiser of the limit functional. In this work we use a natural action functional that is related to the large deviations of a random particle system; the value at each vertex can then be interpreted as a particle density. Our central idea is to study densities as well as particle fluxes through each edge. This additional information allows a full decomposition of the graph into separate components, which simplifies the Γ-convergence considerably.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 08.01.2020, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Dr. Dominik Liebl, Universität Bonn:
Fast and fair simultaneous confidence bands for functional parameters
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Quantifying uncertainty using confidence regions is a central goal of statistical inference. Despite this, methodologies for confidence bands in Functional Data Analysis are underdeveloped compared to estimation and hypothesis testing. This work represents a major leap forward in this area by presenting a new methodology for constructing simultaneous confidence bands for functional parameter estimates. These bands possess a number of striking qualities: (1) they have a nearly closed-form expression, (2) they give nearly exact coverage, (3) they have a finite sample correction, (4) they do not require an estimate of the full covariance of the parameter estimate, and (5) they can be constructed adaptively according to a desired criteria. One option for choosing bands we find especially interesting is the concept of fair bands which allows us to do fair (or equitable) inference over subintervals and could be especially useful in longitudinal studies over long time scales. Our bands are constructed by integrating and extending tools from Random Field Theory, an area that has yet to overlap with Functional Data Analysis. Authors: Dominik Liebl (University Bonn) and Matthew Reimherr (Penn State University).

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Wednesday, 18.12.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Jürgen Sprekels, WIAS Berlin, Humboldt-Universität zu Berlin:
Optimal control of a Cahn--Hilliard--Darcy system with mass source modeling tumor growth
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen”

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 18.12.2019, 11:30 (WIAS-406)
Seminar Interacting Random Systems
Siragan Gailus, Hausdorff Institute for Mathematics/Boston University:
Homogenization of multiscale diffusion processes with small noise
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
Dynamical systems exhibiting multiple characteristic scales in space or time arise naturally as models in a great variety of applied fields. It is moreover common to incorporate random perturbations into these models in order to account for imperfect information or to capture random phenomena. We will discuss homogenization and fluctuations limit theorems for a model of diffusion type. Results on statistical inference will be introduced if time permits.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Monday, 16.12.2019, 14:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
André Uschmajew, MPI Leipzig:
Optimization on low-rank manifolds
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Low-rank matrix and tensor models are important in many applications for representing and embedding high-dimensional data or functions. They typically lead to non-convex optimization problems on sets of matrices or tensors of given rank. In this talk, we give a basic introduction to the geometry of such sets and how it can be used to derive and study optimization algorithms. Compared to direct optimization of the factors in the model, the geometric approach is more intrinsic and can lead to improved methods. For a class of quadratic cost functions on matrices we also discuss how the geometric viewpoint is useful for studying the non-convex optimization landscape under low-rank constraints.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Thursday, 12.12.2019, 16:00 (WIAS-ESH)
Seminar Laserdynamik
Denis S. Goldobin, Perm State University, Russia:
Dynamics of oscillator populations beyond the Ott--Antonsen ansatz
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Thursday, 12.12.2019, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Nancy Hitschfeld Kahler, Universdad de Chile, Santiago:
GPU computing and meshing
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
GPUs are an efficient and low cost alternative to CPU clusters for solving problems that are data-parallel or close to data parallel, but there are some restrictions in the current GPU hardware that must be taken into account in order to get efficient solutions. In this talk, fundamental concepts of GPU computing will be first introduced along with relevant techniques to make optimal use of GPU hardware, such as thread branching, coalesced memory, effective use of shared memory, thread mappings onto a triangular mesh, dynamic memory allocation, thread-safe exclusion mechanisms for neighboring triangles, and indeterminate decisions and traversals over the discrete graph structure of unstructured meshes. Then, an algorithm to transform any triangulation into a Delaunay triangulation and an implementation of a particle tracking algorithm that uses previous algorithm will be discussed. Finally, other relevant aspects to take into account such as gpu-mapping techniques and the ongoing work will be mentioned.

Host
WIAS Berlin
Wednesday, 11.12.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Laura Lauerbach, Universität Würzburg:
Fracture and stochastic homogenization in the passage from discrete to continuous systems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We consider a one dimensional particle system, interacting through nearest-neighbour Lennard--Jones potentials. The non-standard growth conditions and the convex-concave structure of the Lennard-Jones interactions allow for fracture. The interaction potentials are assumed to be randomly distributed. We study the variational limit in the framework of Γ-convergence of this chain of particles which leads to a homogenized energy density. Further, we rescale our model which results in a limiting energy of Griffith's type, consisting of an elastic part and a jump contributions. In a further approach, we study fracture at the level of the discrete energies. This is joint work with S. Neukamm (TU Dresden), M. Schäffner (Leipzig), A. Schlömerkemper (Würzburg).

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 11.12.2019, 11:30 (WIAS-406)
Seminar Interacting Random Systems
Benjamin Lees, University of Bristol:
The phase transition for random loop models on trees
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We show the existence of a sharp phase transition from non-existence to existence of infinite loops for a random loop model on d-regular trees, for all dimensions d ≥ 3. The loop model is built up by randomly placed 'crosses' and 'bars' whose relative intensity is controlled by a parameter u. We give a recursive scheme to obtain an expansion of the critical parameter in powers of 1/d, which in principle is explicit but whose combinatorial complexity grows very quickly. We were able to explicitly obtain the first 6 terms (the first two were previously found by Ueltschi and Bjornberg for the limit d → ∞), and observed that (as functions of u) they seem to have a very interesting structure. This is a joint work with Volker Betz and Johannes Ehlert.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Tuesday, 10.12.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Valeriy Avanesov, WIAS Berlin:
How to gamble with non-stationary X-armed bandits and have no regrets
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
In X-armed bandit problem an agent sequentially interacts with environment which yields a reward based on the vector input the agent provides. The agent's goal is to maximise the sum of these rewards across some number of time steps. The problem and its variations have been a subject of numerous studies, suggesting sub-linear and sometimes optimal strategies. The given paper introduces a new variation of the problem. We consider an environment, which can abruptly change its behaviour an unknown number of times. To that end we propose a novel strategy and prove it attains sub-linear cumulative regret. Moreover, the obtained regret bound matches the best known bound for GP-UCB for a stationary case, and approaches the minimax lower bound in case of highly smooth relation between an action and the corresponding reward. The theoretical result is supported by experimental study.

Host
WIAS Berlin
Wednesday, 04.12.2019, 10:00 (WIAS-405-406)
Forschungsseminar Mathematische Statistik
Prof. Nils Bertschinger, Goethe Universität Frankfurt a. M.:
Systemic Greeks: Measuring risk in financial networks
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
Since the latest financial crisis, the idea of systemic risk has received considerable interest. In particular, contagion effects arising from cross-holdings between interconnected financial firms have been studied extensively. Drawing inspiration from the field of complex networks, these attempts are largely unaware of models and theories for credit risk of individual firms. Here, we note that recent network valuation models extend the seminal structural risk model of Merton (1974). Furthermore, we formally compute sensitivities to various risk factors -- commonly known as Greeks -- in a network context. In the end, we present some numerical illustrations and discuss possible implications for measuring systemic risk as well as insurance pricing.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 03.12.2019, 13:30 (WIAS-406)
Seminar Materialmodellierung
Prof. Michal Pavelka, Charles University, Prague, Tschechische Republik:
Symmetric Hyperbolic Thermodynamically Compatible (SHTC) equations within GENERIC
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Further Informations
Seminar Materialmodellierung

Host
WIAS Berlin
December 3 – 5, 2019 (WIAS-ESH)
Workshop/Konferenz: Workshop on Mathematics of Deep Learning
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Thursday, 28.11.2019, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Markus Kantner, WIAS Berlin:
Modeling of semiconductor devices: From quantum mechanics to non-equilibrium thermodynamics
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Wednesday, 27.11.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Tomáš Roubíček, Charles University Prague, Tschechische Republik:
Fully convective models of some processes in the Earth
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Devised towards geophysical applications for modeling of the solid Earth, a model of poro-elastodynamics with inelastic strains and with convection/diffusion of water will be formulated fully in the Eulerian setting. There, concepts of gradient of the total strain rate as well as the additive splitting of the total strain rate are used, eliminating the displacement from the formulation. It relies on that the elastic strain is small while the inelastic and the total strains can be large. The energetics behind this model is derived and used for analysis as far as the existence of global weak energy-conserving solutions concerns. In some aspects, it improves a model of V. Lyakhovsky at al. to make it thermodynamically consistent and amenable for analysis. Coupling with the fluidic parts of the Earth is also possible while using the concept of elastic (so-called semi-compressible) fluids. Also magnetic phenomena will be discussed both in the solid and the fluidic parts, i.e. paleomagnetism and Earth dynamo, respectively. The talk reflects a collaboration with Giuseppe Tomassetti (Univ. Roma Tre).

Further Informations
Berliner Oberseminar Nichtlineare partielle Differentialgleichungen (Langenbach-Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 27.11.2019, 14:00 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Johannes Zimmer, University of Bath, GB:
Regularisation and analysis of Dean--Kawasaki-type equations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The Dean--Kawasaki-model consists of a nonlinear stochastic partial differential equation describing the evolution of the density function for a system of finitely many particles governed by Langevin dynamics. This equation is formally obtained, in a Schwartz distribution setting, on the hydrodynamic scale. As motivation for the study of this class of equations, we will show that the fluctuations they describe can, in the purely diffusive case, be linked to macroscopic diffusion operators of Wasserstein type. We then derive and analyse a suitably regularised Dean--Kawasaki-model for noninteracting particles obeying a second order Langevin equation, in one space dimension. We prove a high-probability result for the existence and uniqueness of mild solutions to this regularised Dean--Kawasaki-model. Extensions to the case of weakly interacting particles will also be described. This is joint work with Federico Cornalba and Tony Shardlow.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 27.11.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Alain Celisse, University of Lille, France:
Kernelized change-points detection procedure
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 26.11.2019, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Jia-Yuan Dai, National Center for Theoretical Sciences, Taiwan:
Hyperbolicity of Ginzburg--Landau vortex solutions
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstract
We prove that each equilibrium of the Ginzburg-Landau equation restricted on the invariant subspace of vortex solutions is hyperbolic, that is, its associated linearization possesses nonzero eigenvalues. This result completely describes the global attractor of vortex solutions, and also yields the Ginzburg-Landau spiral waves of nodal type. This result is a joint work with Dr. Lappicy.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 26.11.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Franz Besold, WIAS Berlin:
Manifold clustering
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Host
WIAS Berlin
Thursday, 21.11.2019, 11:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Jo Andrea Brüggemann, WIAS:
On the existence of solutions and solution methods for elliptic obstacle-type quasi-variational inequalities with volume constraints
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this talk, an elliptic obstacle-type quasi-variational inequality (QVI) with volume constraints is studied. This type of QVI is motivated by the reformulation of a compliant obstacle problem, where two elastic membranes are subject to external forces while enclosing a constant volume. The existence of solutions to this QVI is established building on fixed-point arguments and partly on the concept of Mosco-convergence. Since Mosco-convergence of the considered feasible sets usually requires complete continuity or compactness properties of the obstacle map, a two-fold approach is explored towards generalising the available existence results for the considered QVI. Based on the analytical findings, the solution of the QVI is approached by solving a sequence of variational inequalities (VIs). Each of these VIs is tackled in function space via a path-following semismooth Newton method. An a posteriori error estimator is derived towards enhancement of the algorithm's numerical performance by using adaptive finite element methods.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 20.11.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Antonius Frederik Maria ter Elst, The University of Auckland, Neuseeland:
The Dirichlet-to-Neumann operator on C 1+κ -domains
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We present some recent results on kernel bounds for the semigroup generated by the Dirichlet-to-Neumann operator when the underlying operator has Hölder continuous coefficients and the domain has a C 1+κ-boundary. The proof depends on Gaussian bounds for derivatives of the semigroup kernel of an elliptic operator with Dirichlet boundary conditions. As a consequence the Dirichlet-to-Neumann semigroup is holomorphic on the right half-plane on L¹. Moreover, it is also strongly continuous on the space of continuous functions on the boundary and holomorphic on the right half-plane.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 20.11.2019, 11:30 (WIAS-HVP-3.13)
Seminar Interacting Random Systems
D. R. Michiel Renger:
Dynamical Phase Transitions on Finite Graphs
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstract
We consider systems where particles jump through edges of a finite graph with nonlinear intensities, and study time-averaged particle fluxes as both the number of particles and the end time go to infinity. The corresponding large-deviation rate functional involve a minimisation over trajectories with a given time-averaged flux. The minimisation can often be restricted to constant paths, resulting in a very simple expression for the rate functional. However, for specific models and specific regimes of the average flux, such expression overshoot the large-deviation rate functional, which typically happens if small oscillations in the trajectories are more profitable. In that case we say that a dynamical phase transition occurs. The literature on dynamical phase transition is almost exclusively restricted to models where the graph becomes continuous in the limit, yielding quadratic rate functionals. In our work we focus on graphs that remain discrete in the limit, which leads to entropic rate functionals. We present conditions that rule out dynamical phase transition, and for zero-range processes on a discrete ring, we give sufficient conditions under which a dynamical phase transition can be constructed. (Joint work with Davide Gabrielli, l'Aquila)

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Wednesday, 20.11.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Nikita Zhivotowskii, Google Zürich, Switzerland:
Robust covariance estimation for vectors with bounded kurtosis
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Let X be a centered random vector and assume that we want to estimate its covariance matrix. In this talk I will discuss the following result: if the random X satisfies the bounded kurtosis assumption, there is a covariance matrix estimator that given a sequence of n independent random vectors distributed according to X exhibits the optimal performance one would expect had X been a gaussian vector. The procedure also improves the current state-of-the-art regarding high probability bounds in the sub-gaussian case (sharp results were only known in expectation or with constant probability). In both scenarios the new bound does not depend explicitly on the dimension, but rather on the effective rank of the covariance matrix of X. The talk is based on the joint work with S. Mendelson "Robust covariance estimation under L4-L2 moment equivalence", to appear in AoS 2019.

Host
WIAS Berlin
Universität Potsdam
Humboldt-Universität zu Berlin
Tuesday, 19.11.2019, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Ralf Toenjes, University of Potsdam:
The constructive role of noise in the dynamics on network hubs for network synchronization
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstract
We describe and analyze a coherence resonance phenomenon for synchronization in bipartite networks of well connected hubs and followers when the hubs are subjected to noise. Using the Ott-Antonsen ansatz for globally coupled phase oscillators the dynamics of the mean fields is described by a low-dimensional system of Langevin equations. Averaging over the fast stochastic dynamics of the hubs yields ordinary differential equations which predict the coherence resonance reasonably well.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 19.11.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Yangwen Sun, Humboldt-Universität zu Berlin:
Online change-point detection for high-dimensional data using graphs
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Host
WIAS Berlin
Wednesday, 13.11.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Michael Kniely, Institute of Science and Technology Austria (IST Austria) , Österreich:
On the large-time behavior of a class of semiconductor equations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
This talk will be concerned with the exponential convergence to equilibrium of solutions to some semiconductor models arising from different modeling perspectives. The resulting PDE systems typically include drift-diffusion terms for electrons and holes as well as reaction terms describing the underlying electron-hole recombination mechanism. Including also the self-consistent electrostatic potential is often desired from a physical point of view. The main goal of the talk is the derivation of functional entropy-entropy dissipation inequalities in a constructive way for a set of semiconductor equations with and without potential and including different reaction processes. To this end, we will revisit nowadays classical calculations for prototype systems, and we shall also investigate more elaborate strategies necessary to deal with more complex models. Some parts of the talk are based on joint work with Klemens Fellner.

Further Informations
Berliner Oberseminar Nichtlineare partielle Differentialgleichungen (Langenbach-Seminar)

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 13.11.2019, 10:00 (WIAS-405-406)
Forschungsseminar Mathematische Statistik
Merle Behr, University of California, Berkeley:
Learning compositional structures
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 12.11.2019, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Prof. Dr. Nicolas Perkovski, Freie Universität Berlin:
From hopping particles to stochastic PDEs
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstract
I will try to give an overview of some of my research interests, focusing on certain a priori ill posed stochastic PDEs and their derivation. An important task in stochastics is to find and construct üniversal" models that describe a given phenomenon. For example, any random variable that is given by the superposition of many small independent influences is approximately Gaussian, independently of the concrete nature of the small influences, and therefore we call the Gaussian distribution universal. When trying to derive universal models for phenomena that evolve in space and time, formal calculations often suggest that we should consider nonlinear stochastic PDEs driven by space-time white noise. This is a problem because due to the irregularity of the noise the solution might be too irregular to make sense of the nonlinearities in the equation. But in recent years we found new ways of overcoming these problems, making sense of the equations, and proving their universality in some cases.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 12.11.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Olga Krivorotko, Russian Academy of Sciences Novosibirsk:
Regularization of multi-parametric inverse problems for differential equations arising in immunology, epidemiology and economy
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Host
WIAS Berlin
Thursday, 07.11.2019, 14:00 (WIAS-405-406)
Seminar Numerische Mathematik
Benoît Gaudeul, Université de Lille , Frankreich:
Some numerical schemes for a reduced case of a Nernst--Planck--Poisson model
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
We compare several di erent numerical strategies to simulate a Nernst--Planck--Poisson model introduced in [1]. Different equivalent formulations are exploited based either on concentrations or activities [2]. As a first step, we focus on a simplified model for ionic liquid, for which four different schemes are discussed. A particular attention is given to the preservation at the discrete level of key features of the continuous problem, that are the positivity of the concentrations and the dissipation of the energy along time. The existence and convergence of a discrete solutions to the nonlinear system corresponding to each scheme is then established. Finally, numerical comparisons between the schemes are provided.

References
[1] Wolfgang Dreyer, Clemens Guhlke, and Rüdiger Müller. Overcoming the shortcomings of the Nernst--Planck model. Physical Chemistry Chemical Physics, 15, 2013.
[2] Jürgen Fuhrmann. Comparison and numerical treatment of generalised Nernst--Planck models. Computer Physics Communications, 6, 2015.

Host
WIAS Berlin
November 6 – 8, 2019 (WIAS-ESH)
Workshop/Konferenz: Foundations of Modern Statistics
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Humboldt-Universität zu Berlin
DFG Research Unit 1735
International Research Training Group 1792
Wednesday, 06.11.2019, 11:30 (WIAS-HVP-3.13)
Seminar Interacting Random Systems
Tal Orenshtein, WIAS:
Random walks in random environment as rough paths
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstract
Random walk in random environment (RWRE) is a model to describe propagation of heat or diffusion of matter through a highly irregular medium. The latter is expressed locally in the model in terms of a random environment according to which the process evolves randomly in time. In a few fundamental classes the phenomenon of homogenization of the media takes place. One way this can be expressed is in the fact that on large scales, the RWRE looks like a Brownian motion with a deterministic covariance matrix given in terms the (law of the) environment. Rough path theory enables the construction of solutions to SDEs so that the solution map is continuous with respect to the noise. One important application guarantees that if the approximation converges to the noise in the rough path topology, the SDEs driven by the noise approximations converge, in an appropriate sense, to a well-defined SDE different than the original one, so that the correction term is explicit in terms of the noise approximation. In this talk we shall present our current program, in which one lifts RWRE in various classes to the rough path space and shows a convergence to an enhanced Brownian motion in the rough path topology. Interestingly, the limiting second level of the lifted RWRE may have a linear correction, called area anomaly, which we identify. Except for the immediate application to approximations of SDEs (and potentially of SPDEs), this adds some new information on the RWRE limiting path. Time permitted, we shall elaborate on the tools to tackle these problems. Based on joint works with Olga Lopusanschi, with Jean-Dominique Deushcel and Nicolas Perkowski and with Johaness Bäumler, Noam Berger, and Martin Slowik.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Wednesday, 06.11.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Charles Manski, North Western University, USA:
Patient care under uncertainty (Hermann Otto Hirschfeld Lecture 2019)
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 05.11.2019, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Phillipo Lappicy, Universidade de Sao Paulo, FU Berlin:
Hořava--Lifshitz gravity: Bifurcations and chaos
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 05.11.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Roman Kravchenko, WIAS und HU Berlin:
Optimal transport for imaging
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Host
WIAS Berlin
Monday, 04.11.2019, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Christopher Rackauckas, Massachusetts Institute of Technology/University of Maryland, USA:
Neural differential equations as a basis for scientific machine learning
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Scientific Machine Learning (SciML) is an emerging discipline which merges the mechanistic models of science and engineering with non-mechanistic machine learning models to solve problems which were previously intractable. Recent results have showcased how methods like Physics Informed Neural Networks (PINNs) can be utilized as a data-efficient learning method, embedding the structure of physical laws as a prior into a learnable structures so that small data and neural networks can sufficiently predict phenomena. Additionally, deep learning embedded within backwards stochastic differential equations has been shown to be an effective tool for solving high-dimensional partial differential equations, like the Hamilton-Jacobian-Bellman equation with 1000 dimensions. In this talk we will introduce the audience to these methods and show how these diverse methods are all instantiations of a neural differential equation, a differential equation where all or part of the equation is described by a latent neural network. Once this is realized, we will show how a computational tool, DiffEqFlux.jl, is being optimized to allow for efficient training of a wide variety of neural differential equations, explaining how the performance properties of these equation differ from more traditional uses of differential equations and some of the early results of optimizing for this domain. The audience will leave knowing how neural differential equations and DiffEqFlux.jl may be a vital part of next-generation scientific tooling.

Host
WIAS Berlin
Friday, 01.11.2019, 14:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Ana Djurdjevac, FU Berlin, Institut für Mathematik:
Random PDEs on moving hypersurfaces
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
It is well-known that in a variety of applications, especially in the biological modeling, PDEs that appear can be better formulated on evolving curved domains. Most of these equations contain various parameters and often there is a degree of uncertainty regarding the given data. We investigate the uncertainty which comes from unknown parameters or geometry. In the first part we study the advection-diffusion equation with random coefficients that is posed on an evolving hypersurface. We consider both cases, uniformly bounded and log-normal distributions of the coefficient. We introduce the solution space and prove the well-posedness using Banach - Nechas - Babushka theorem. Furthermore, we will introduce and analyse the evolving surface finite element discretization of the equation, introduced by Dziuk and Elliott. In the uniformly bounded case, we show unique solvability of the resulting semi-discrete problem and prove optimal error bounds for the semi-discrete solution and Monte Carlo of its expectation. In the second part we study PDEs that evolve with a given random velocity. Utilizing the domain mapping method, we transfer the problem into a PDE with random coefficients on a fixed domain and analyse this equation. Our theoretical convergence rates are confirmed by numerical experiments. This work is supported by DFG through project AA1-3 of MATH+

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Thursday, 31.10.2019, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Jan-Philipp Köster, Ferdinand-Braun-Institut Berlin:
Traveling wave model based simulation of tunable multi-wavelength diode laser and MOPA systems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Thursday, 31.10.2019, 14:00 (WIAS-ESH)
INSTITUTSKOLLOQUIUM
Prof. Max Gunzburger, Florida State University, USA:
Four “better” ways to solve the Navier--Stokes equations: Simulation of Richardson pair dispersion, ensemble discretization methods, an auxiliary equation approach for UQ, and filtered regularizations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The facetious and self-serving title refers to four approaches for Navier-Stokes simulations. The first involves the analysis, numerical analysis, and an efficient implementation strategy for a recently proposed fractional Laplacian closure model that accounts for Richardson pair dispersion observed in turbulent flows. The second is the exploitation of accurate and widely applicable ensemble methods in settings in which multiple inputs need to be processed, as may be the case for uncertainty quantification, reduced-order modeling, and control and optimization. The third addresses the lack of regularity of solutions and the resultant loss of accuracy of approximations in the case of white or weakly correlated additive noise forcing. The fourth involves filtered spectral viscosity and hierarchical finite element methods for regularized Navier-Stokes equations.

Further Informations
Institutskolloquium

Host
WIAS Berlin
Thursday, 31.10.2019, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Grigor Nika, WIAS Berlin:
Optimal design of 3D-printed microarchitectured polymer materials exhibiting a negative Poisson's ratio
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Wednesday, 30.10.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Katharina Hopf, WIAS Berlin:
On the singularity formation and relaxation to equilibrium in 1D Fokker--Planck model with superlinear drift
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 30.10.2019, 11:30 (WIAS-406)
Seminar Interacting Random Systems
Dr. Niccolo Torri, Université Paris Nanterre, Frankreich:
Directed polymer in a heavy-tail random environment
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
A classical problem in the field of disordered systems is to understand the behaviour of a directed polymer in interaction with a random environment. Mathematically, the directed polymer is modeled by a directed random walk and the random environment is a sequence of random variables which can interact with the random walk perturbing its behaviour, giving rise to super-diffusive transversal fluctuations and localisation phenomena. Understanding the typical trajectories of the the walk is a very challenging issue and several results have been obtained when the random environment has exponential moments. In this talk we consider the case of a heavy-tail random environment, that is, the environment?s distribution function decades polynomially with exponent $alpha >0$. We mainly focus on the case in which the random environment has no second moment ($alphain (0,2)$), finding the super-diffusive transversal fluctuations of the polymer as function of the parameter $alpha$. Joint work with Quentin Berger (Sorbonne Université).

Further Informations
Seminar Interacting Random Suystems

Host
WIAS Berlin
Tuesday, 29.10.2019, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Pavel Gurevich and Hannes Stuke, Freie Universität Berlin:
Reinforcement learning
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 29.10.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Tim Jahn, Universität Frankfurt am Main:
Beyond the Bakushinskii veto
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Host
WIAS Berlin
Wednesday, 23.10.2019, 14:00 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Ansgar Jüngel, Technische Universität Wien, Österreich:
Cross-diffusion systems: from spin semiconductors to biological populations with stochastic forcing
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Many real-world applications consist of multiple components, leading on the macroscopic scale to cross-diffusion systems which consist of strongly coupled parabolic equations. The applications may be very diverse and range from spin-polarized transport in semiconductors and ion transport in cell membranes to population dynamics. In this talk, some existence results for global-in-time weak solutions is proved, based on entropy methods. This technique was already used by Herbert Gajewski, and we detail some of his ideas to prove the large-time behavior and uniqueness of weak solutions, followed by some extensions like boundedness-by-entropy and martingale solutions to stochastic cross-diffusion systems.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 23.10.2019, 11:00 (WIAS-HVP-3.13)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Martin Holler, Universität Graz, Österreich:
A variational model for learning convolutional image atoms from incomplete data
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstract
Using lifting and relaxation strategies, we present a convex variational model for learning a convolutional sparse representation of image data via a few basic atoms. Such a representation provides a model for repeating patterns, but is also of interest for classification or as structural prior. We ensure well-posedness results for the proposed model in a general inverse problems setting and provide numerical experiments, where an atom-based representation is computed from incomplete, noisy and blurry data.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 23.10.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Vladimir Spokoiny, WIAS Berlin, HU Berlin:
Bayesian inference for nonlinear inverse problems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We discuss the properties of the posterior for a wide class of statistical models including nonlinear generalised regression and deep neuronal networks, nonlinear inverse problems, nonparametric diffusion, error-in-operator and IV models. The new calming approach helps to treat all such problems in a unified manner and to obtain tight finite sample results about Gaussian approximation of the posterior with an explicit error bound in term of so called effective dimension.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 22.10.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Oleg Butkovsky, WIAS Berlin:
Regularization by noise for SDEs and SPDEs with applications to numerical methods
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Host
WIAS Berlin
Wednesday, 16.10.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Joachim Rehberg, WIAS Berlin:
Explicit and uniform estimates for second order divergence operators on Lp spaces
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 16.10.2019, 11:30 (WIAS-406)
Seminar Interacting Random Systems
Dr. Christian Mönch, Johannes-Gutenberg-Universität:
Universality of persistence exponents for self-similar processes with stationary increments
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
n 1999, G. Molchan showed that for a centered fractional Brownian motion X on the real line Prob[X(t) <1, 1 <t<T]=TH-1+o(1), where H is the index of self-similarity of X. Furthermore, he showed that the same tail exponent occurs for sevaral other path functionals of X and conjectured that it also governs the tail of Prob[L(t)<1, 1<t<T], where L is the local time at 0 of X, but was only able to prove a lower bound. In this talk, I present an entirely novel approach to persistence problems for self-similar processes with stationary increments based on Palm theory. The technique is not limited to Gaussian processes, allows us to resolve Molchan's conjecture for ANY H-self-similar process with stationary increments that admits a sufficiently smooth local time and provides a better error estimate even for the known lower bound of Prob[L(t)<1, 1<t<T] in the fractional Brownian case. Furhermore, I will discuss some work in progress concerning fractional variants of Brownian excursion-type processes.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
DFG Research Center MATHEON
Wednesday, 16.10.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Alexey Onatskiy, Cambridge University:
Cointegration the modern way
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 15.10.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Alexandra Suvorikova, Universität Potsdam:
Shape-based domain adaptation
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Host
WIAS Berlin
Wednesday, 09.10.2019, 11:30 (WIAS-406)
Seminar Interacting Random Systems
Dr. Lorenzo Taggi, WIAS:
Phase transition in random lattice permutations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We consider the model of random lattice permutations, whose realisations are permutations of the vertices of Z^d and can be viewed as a system of oriented self-avoiding loops interacting via mutual exclusion, with parameter beta (”inverse temperature”) which rewards the number ”jumps”. This model attracts interest from different perspectives: it is related to the interacting quantum Bose gas through a representation which is due to Feynman, it can be viewed as representation of the dimer model (when the inverse temperature is infinite) and it is a variant of loop O(N) models. A central question for all these models is whether a regime of ”macroscopic loops” and uniformly positive "two point correlations" occurs when the inverse temperature is large enough and d > 2. This talk presents the results and the proof techniques of a recent paper where a positive answer to this question is provided.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Tuesday, 08.10.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexander Gasnikov, Moscow Institute of Physics and Technology:
Introduction lecture in non convex optimization
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
In this talk we try to observe the modern numerical methods for non convex optimization problems. In particular, we concentrate on gradient type methods, high-order methods and randomization of sum type methods. Most of the results are taken from the papers that were published in the last 2-3 years.

Host
WIAS Berlin
Monday, 07.10.2019, 14:00 (WIAS-ESH)
Colloquium on the occasion of the 70th birthday of Dr. Johannes Elschner


Prof. Dr. Masahiro Yamamoto (University of Tokyo):
Inverse problems for first-order transport equations by Carleman estimates

Prof. Dr. Guang-Hui Hu (Beijing Computational Science Research Center):
Corner scattering theory and data-driven inversion schemes
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin

Thursday, 26.09.2019, 15:00 (WIAS-406)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Prof. Dr. Martin Brokate, Technische Universität München:
Sensitivity in rate independent evolutions
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
As a topic in science, rate independent evolutions have appeared more than 100 years ago; their study as a mathematical subject in its own began in the 1960's. We will present some basic issues and then discuss in particular the question of differential sensitivity, that is, whether the associated solution operators possess weak derivatives.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
WIAS Berlin
September 18 – 20, 2019 (WIAS-ESH)
Workshop/Konferenz: Optical Solitons and Frequency Comb Generation
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
September 9 – 13, 2019 (WIAS-ESH)
Workshop/Konferenz: PDE 2019: Partial Differential Equations in Fluids and Solids
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
This workshop fuses expertise on the analysis of PDEs in solids, fluid dynamics, complex fluids, and interaction of fluids with solid structure. Its goal is to create new synergies among these fields in order to advance analytical methods for current research related to systems with bulk-interface interaction, geometrically nonlinear materials, and fluid-structure interaction. This concerns, e. g., Lagrangian and Eulerian descriptions, free boundaries and moving domains, and variational approaches via energy/entropy methods.

Host
WIAS Berlin
Wednesday, 04.09.2019, 11:30 (WIAS-406)
Seminar Interacting Random Systems
Alexander Hinsen, WIAS:
Phase transition for the white knight model
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Tuesday, 03.09.2019, 13:30 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Yori Fournier, Leibniz-Institut für Astrophysik Potsdam (AIP):
Non-local effects in the solar dynamo
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Model of the solar dynamo hardly reproduce the observations of the solar magnetic field. In the presence of strong magnetic field in a turbulent fully ionized medium, such as the solar interior, temporal and spacial re-correlations can occur. In the frame of mean-field MHD, these can be treated as spacial and temporal non-localities (memory effect). We found that the magnetic buoyancy occurring in the convective solar interior implies a non-linear temporal non-locality. This additional non-linearity leads to solutions that recover the morphological evolution of the solar magnetic field.

Host
WIAS Berlin
Monday, 19.08.2019, 15:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Emil Vosmar Denning, Technical University of Denmark:
Accurate few-mode models of highly structured photonic reservoirs
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
August 3 – 8, 2019 (TU-Berlin)
Workshop/Konferenz: 6th International Conference on Continuous Optimization
more ... Location
Technische Universität, Straße des 17. Juni 135, 10623 Berlin, Main building (H)

Further Informations
https://iccopt2019.berlin/

Host
WIAS Berlin
Technische Universität Berlin
Freie Universität Berlin
Humboldt-Universität zu Berlin
Mathematical Optimization Society
Friday, 02.08.2019, 15:15 (WIAS-ESH)
MATHEON Special Guest Lecture
Prof. Ya-Xiang Yuan, Chinese Academy of Sciences, China:
Efficient optimization algorithms for large scale data analysis
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this talk, two classes of problems in large scale data analysis and their optimization algorithms will be discussed. The first class focuses on composite convex program problems, where I introduce algorithms including a regularized semi-smooth Newton method, a stochastic semi-smooth Newton method and a parallel subspace correction method. The second class is on optimization with orthogonality constraints, particularly on parallelizable approaches for linear eigenvalue problems and nonlinear eigenvalue problems, and quasi-Newton type methods. Numerical results of applications, e.g., electronic structure calculations, $l_1$-regularized logistic regression problems, Lasso problems and Hartree-Fock total energy minimization problems, will be highlighted.

Further Informations
Special Guest Lecture

Host
WIAS Berlin
Tuesday, 30.07.2019, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Jonathan Trantow, Technische Universität Berlin:
A pseudo time-stepping approach for the evolution of elastic curves
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Thursday, 25.07.2019, 10:00 (WIAS-406)
Seminar Materialmodellierung
Dr. Robert Style, ETH Zürich, Schweiz:
Arresting phase separation with polymer networks
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
Some of the most beautiful colours in nature are seen in birds that have developed materials with extremely monodisperse, colloidal microstructures (these yield vivid structural colours). Previously it has been suggested that these can be grown by a process of arrested phase separation. Here, we take inspiration from such natural materials to grow composites with a uniform microstructure via a process of phase separation in an elastic gel network. These composites consist of uniform liquid droplets embedded in an elastic gel. The size of the droplets can be easily tuned with a number of different parameters, and presents an interesting challenge for modelling. I will also discuss how this process has applications in colloidal synthesis and phase-separation processes in living cells.

Host
WIAS Berlin
Friday, 12.07.2019, 11:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Toshiyuki Ogawa, Meiji University, Tokyo, Japan:
Drift bifurcation of traveling waves in a reaction-diffusion system with 3 competing species
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Host
Freie Universität Berlin
WIAS Berlin
Thursday, 11.07.2019, 15:00 (WIAS-405-406)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Georg Stadler, New York University, USA:
Sparse optimal control of PDEs with uncertain coefficients
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
I will discuss sparse solutions of optimal control problems governed by elliptic PDEs with uncertain coefficients. Sparsity of controls is achieved by incorporating the L^1-norm of the mean of the pointwise squared controls in the objective. The main focus is on stochastic controls that share the same sparsity structure, i.e., controls that depend on the realization of the random parameters but have identical support. We propose an iterative norm reweighting formulation, which iterates over functions defined over the physical space only and thus avoids approximation of the random space. Combining a Newton method with low-rank operator approximations, this results in an efficient solution method that avoids approximation of the uncertain parameter random space. The qualitative structure of the optimal controls and the performance of the solution algorithm are studied numerically using control problems governed by the Laplace and Helmholtz equations. This is joint work with Chen Li (NYU).

Host
WIAS Berlin
Thursday, 11.07.2019, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Arijit Hazra, Tata Institute of Fundamental Research, Indien:
Globally constraint-preserving FR/DG scheme for Maxwell's equations up to fifth order of accuracy
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Computational electrodynamics (CED), the numerical solution of Maxwell?s equations, plays an incredibly important role in several problems in science and engineering. High accuracy solutions are desired, and the discontinuous Galerkin (DG)-based methods is one of the most sophisticated ways of delivering high accuracy in numerical CED. Maxwell?s equations also have a pair of involution constraints and mimetic schemes that globally satisfy the constraints at a discrete level are also highly desirable. Keeping the above desirable properties in mind, in this work, we propose a globally divergence-conforming DG-like scheme for CED upto fifth orders of accuracy. We achieve global constraint-preservation of involution constraints of CED by collocating the electric displacement, magnetic induction and their higher order modes in the faces of the mesh. In our numerical scheme, one has to evolve some zone-centered modes in addition to the face-centered modes at fourth and higher orders of accuracy. The novel features of our schemes are: retention of higher order accuracy without any limiter even when permittivity and permeability vary by almost an order of magnitude, superior control of numerical diffusion and preservation of electromagnetic energy with strongly varying material properties in absence of conductivity. Please do not hesitate to contact me if you have any further queries. I would like to thank you for you understanding and cooperation.

Host
WIAS Berlin
Wednesday, 10.07.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Artur Stephan, WIAS Berlin - BMS:
On EDP--convergence for linear reaction-diffusion systems with different time scales
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen" (Langenbach-Seminar)

Host
WIAS Berlin
Berlin Mathematical School
Wednesday, 10.07.2019, 13:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Georg Stadler, New York University, USA:
Estimation of extreme event probabilities by combining large deviation theory and PDE--constrained optimization, with application to tsunami waves
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Tsunami waves are caused by a sudden change of ocean depth (bathymetry) after an earthquake below the ocean floor. Since large tsunami waves are extreme events, they correspond to the tail part of a probability distribution, whose exploration would require impractically many samples of a Monte Carlo method. We propose an alternative method to estimate extreme probabilities using large deviation theory (LDT), which relates the probabilities of extreme events to the solutions of a one-parameter family of optimization problems. We model tsunami waves with the shallow water equations and thus these equations appear as PDE-constraints in this optimization problem. The optimization objective includes a term that measures how extreme the event is, and a term corresponding to the likelihood of certain bathymetry changes, which are modeled as a Gaussian random field. Preliminary numerical results with the 1D inviscid shallow water equation are presented. This is joint work with Shanyin Tong and Eric Vanden-Eijnden (both NYU).

Host
WIAS Berlin
Wednesday, 10.07.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Arnak Dalalyan, ENSAE ParisTech:
Robust estimation in structured linear models by convex programming
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 09.07.2019, 13:30 (WIAS-406)
Seminar Materialmodellierung
Prof. Carsten Gräser, Freie Universität Berlin, Institut für Mathematik:
Truncated nonsmooth Newton multigrid for nonsmooth minimization problems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
Many problems originating from continuum mechanics and material sciencelead to large scale nonsmooth optimization problems after discretization in time and space. Examples are classical binary or multi-component phase field models for phase transition and separation, frictional contact problems, plasticity, and phase field-like approaches for brittle and ductile fracture. Since standard numerical methods like, e.g., multigrid are not directly applicable due to the nonsmoothness, generic nonsmooth optimization methods are frequently used for such problems which often comes at the price of reduced efficiency. In the talk we present the Truncated Nonsmooth Newton Multigrid (TNNMG) method which combines techniques from nonsmooth optimization with multigrid and domain decomposition ideas. Instead of a black box approach this is done in a structure aware fashion leading to iterative methods whose efficiency is comparable to state of the art methods for smooth problems while being robust with respect nonsmoothness. In the talk we will introduce the algorithm, discuss convergence, and present numerical examples for various applications illustrating the efficiency of the presented approach.

Host
WIAS Berlin
Wednesday, 03.07.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Pierre-Étienne Druet, WIAS Berlin:
Weak solution analysis for incompressible multicomponent flow models
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen" (Langenbach-Seminar)

Host
WIAS Berlin
Wednesday, 03.07.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Claudia Strauch, Universität Mannheim:
Concentration and nonparametric learning of diffusion processes
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We start by discussing uniform concentration inequalities for continuous-time analogues of empirical processes and related stochastic integrals of ergodic diffusion processes. Our approach substantially relies on combining the device of martingale approximation and moment bounds which are obtained by the generic chaining method. As a concrete statistical application, we consider the question of estimating the drift function for a large class of ergodic diffusion processes. The unknown drift is supposed to belong to a nonparametric class of smooth functions of unknown order. We suggest a fully data-driven procedure which allows for rate-optimal drift estimation (with respect to sup-norm risk) and, at the same time, yields an asymptotically effiient estimator of the invariant density of the diffusion. In the last part of the talk, we sketch applications of our results to problems from stochastic control theory. One of the fundamental assumptions in stochastic control of continuous time processes is that the dynamics of the underlying (diffusion) process is known. This is, however, usually not fulfilled in practice. We study a toy model for harvesting and natural resource management, mathematically described as an impulse control problem. In variants of this model, we suggest ways to both learn the dynamics of the underlying process and control well at the same time. In particular, the combination of results from stochastic control and our previous analysis of the sup-norm risk allows to derive mathematical results for reinforcement learning.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 02.07.2019, 13:30 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Ankik Kumar Giri, Indian Institute of Technology Roorkee:
Recent developments in the theory of coagulation-fragmentation models
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The coagulation-fragmentation equations describe the kinetics of particle growth in which particles can coagulate via binary interaction to form larger particles or fragment to form smaller ones. These models arise in many fields of science and engineering: kinetic of phase transformations in binary alloys such as segregation of binary alloys, aggregation of red blood cells in biology, fluidized bed granulation processes, raindrop breakup in clouds, aerosol physics, i.e. the evolution of a system of solid or liquid particles suspended in a gas, formation of planets in astrophysics, polymer science and many more.
The coagulation is in general a nonlinear process where the fragmentation is classified into two major categories, one of them is the linear fragmentation and another one is the nonlinear fragmentation. The linear fragmentation may occur due to the external forces or spontaneously (that depends on the nature of particles). However, the nonlinear fragmentation takes place due to the collision between a pair of particles. Therefore, it is also known as collision-induced fragmentation or collisional breakage.
In general, the non-conservative approximation of coagulation and linear fragmentation equations may lead to the occurrence of gelation phenomenon i.e. the breakdown of mass conservation property of the solution. In the first half of this talk, it is shown that the non-conservative approximation of coagulation and linear fragmentation equations can also provide the existence of mass conserving solutions for large classes of unbounded coagulation and fragmentation kernels.The fragmentation kernel may have a singularity near the origin. Later on, this result is further generalized by including the singular coagulation kernels in the existence theory of mass-conserving solution to the nonlinear coagulation equation using non-conservative approximations.
In the second half, we introduce an existence result on weak solutions to the continuous coagulation equation with collision-induced fragmentation for certain classes of unbounded collision and breakup distribution kernels. The breakup kernel may have a possibility to attain a singularity at the origin. The proof is based on the weak compactness methods applied to suitably chosen conservative approximating equations. The question of uniqueness is also considered under additional growth conditions on the kernels which mainly relies on the integrability of higher moments. Moreover, It is observed that the unique solution is mass-conserving.

Host
WIAS Berlin
Thursday, 27.06.2019, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Markus Kantner, WIAS Berlin:
Non-isothermal generalization of the Scharfetter--Gummel scheme for degenerate semiconductors
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Wednesday, 26.06.2019, 10:00 (WIAS-405-406)
Forschungsseminar Mathematische Statistik
Dr. Alexandra Suvorikova, Universität Potsdam:
On CLT in Bures-Wasserstein space and beyond
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
In the first part of the talk we present some concentration and convergence properties of Bures-Wasserstein (BW) barycenters of hermitian finite-dimensional matrices, and explain how they can be used for investigation of geometry of DNA molecules modelled as a union of ridgid bodies. In the second part we show how the framework of classical resampling techniques can be extended to the case of the BW space, and introduce some geometrical intuition behind the construction of non-asymptotic confidence sets for BW barycenters.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 25.06.2019, 15:15 (WIAS-HVP-3.13)
Oberseminar Nonlinear Dynamics
Stefan Ruschel, TU Berlin:
Pulsing dynamics of a laser system with delayed feedback
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstract
We consider the Yamada model for an excitable or self-pulsating laser with saturable absorber in the presence of delayed optical self-feedback. We present some recent results regarding the stability and bifurcations of periodic light pulses that are sustained in the external cavity feedback loop, so called external cavity pulse trains.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 25.06.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Fabian Telschow, University of California San Diego, USA:
Spatial confidence sets for raw effect size images
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Host
WIAS Berlin
Tuesday, 25.06.2019, 13:30 (WIAS-406)
Seminar Materialmodellierung
Prof. Luca Heltai:
Unconventional frameworks for the simulation of coupled bulk-interface problems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
Partial differential equations with interfaces, holes, cracks, or defects often require the numerical solution of coupled bulk-interface problems. In this talk, I will discuss and analyse some techniques that can be used to tackle this class of problems, using non-matching discretisations that combine finite element methods, regularization techniques, weighted Sobolev spaces, and reduce order models.

Host
WIAS Berlin
Monday, 24.06.2019, 15:15 (WIAS-HVP-3.13)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Jens Gravesen, Technical University of Denmark, Dänemark:
Differential geometry and dimension reduction for nano structures
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstract
Many nano structures are essentially one or two dimensional and it is in this case often possible to reduce the equations from three to one or two dimensions. That gives huge computational savings but can also provide insight into how the geometry of structures influences the physical properties. In the case of the Schrödinger equation for a single particle this influence is expressed in the form of a simple curvature potential. For coupled equations such as the Luttinger--Kohn Hamiltonian the procedure is no longer straight forward. The boundary conditions pose some problems and an extra approximation is necessary. We will give an overview of the method and point to some of the possible pitfalls when a choice of frame is needed.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
June 24 – 26, 2019 (WIAS-ESH)
Workshop/Konferenz: Workshop on Phase Transitions and Particle Systems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Wednesday, 19.06.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Janusz Ginster, Technische Universität Berlin:
Plasticity as the Γ-limit of models for straight, parallel dislocations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Crystal plasticity is the effect of a crystal undergoing a permanent change of shape in response to applied forces. At the atomic scale, dislocations - local defects of the crystalline lattice concentrated on lines - are considered to play a main role in this effect. In this talk, we will derive a macroscopic strain-gradient plasticity theory from two different semi-discrete models for straight, parallel edge dislocations. Firstly, we complement the existing literature by considering a rotationally invariant energy with mixed growth. The main advantage of this formulation is that it does not need the ad-hoc definition of a core-radius. In addition to the Γ-convergence result we present a rigidity result for fields with non-vanishing curl in this setting. Secondly, we will address the fact that most results in the literature are derived under the assumption that the present dislocations are separated on a mesoscopic scale. We will show how to overcome this assumption by an adaption of the ball construction technique which was developed in the context of the Ginzburg--Landau model for superconductivity.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 19.06.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Harel Hron, Palacký University, Olomouc, Czech Republic:
Weighting of densities in Bayes spaces with application to simplicial functional principal component analysis
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Probability density functions (PDFs) can be understood as functional data carrying relative information. As such, standard methods of functional data analysis (FDA) are not appropriate for their statistical processing. They are typically designed in the L2 space (with Lebesgue reference measure), thus cannot be directly applied to densities, as the metrics of L2 does not honor their geometric properties. This has recently motivated the construction of the so-called Bayes Hilbert spaces, which result from the generalization of the Aitchison geometry for compositional data to the in nite dimensional setting. More precisely, if we focus on PDFs restricted to a bounded support (that is mostly used in practical applications), they can be represented with respect to the Lebesgue reference measure using the Bayes space of positive real functions with square-integrable logarithm. The reference measure can be easily changed through the well-known chain rule and interpreted as a weighting technique in Bayes spaces. Moreover, it impacts on the geometry of the Bayes spaces and results in so-called weighted Bayes spaces. The aim of this contribution is to show the e ects of changing the reference measure from the Lebesgue measure to a general probability measure focusing on its practical implications for the Simplicial Functional Principal Component Analysis (SFPCA). A centered log-ratio transformation is proposed to map a weighted Bayes spaces into an unweighted L2 space (i.e. with Lebesgue reference measure), thus it enables to apply standard statistical methods such as SFPCA on PDFs.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 18.06.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Priv.-Doz. Dr. Peter Mathé, WIAS Berlin:
The modulus of continuity in Bayesian inverse problems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
We shall discuss how the convergence rates of direct and inverse problems are related through the modulus of continuity.

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Tuesday, 18.06.2019, 13:30 (WIAS-406)
Seminar Materialmodellierung
Prof. Amit Acharya, Carnegie Mellon University, USA:
Line defect dynamics and solid mechanics
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
Continuum mechanics has been a successful model for studying macroscopic deformations and the forces causing them. The usual framework allows the study of continuous deformations giving way to surfaces of discontinuity, but does not provide an adequate framework for considering the dynamics of the terminating lines of surfaces of discontinuity, were such to occur. It turns out that such terminating lines of surfaces of discontinuity serve as a model of common line defects that arise in a host of materials; dislocations and grain/phase boundary junctions in crystalline and soft matter. I will describe a framework for considering line defect dynamics within continuum mechanics. I will show how the kinematics of line defect dynamics provides a unifying theme for describing the defects mentioned above, resulting in an augmentation of the classical balance laws of continuum mechanics with a microscopic conservation law for topological charge carried by these defect lines. The theory will be illustrated with examples related to dislocation dynamics with inertia, the computation of fields of interfacial defects like the star disclination and grain boundary disconnections.

Further Informations
Seminar Materialmodellierung

Host
WIAS Berlin
Wednesday, 12.06.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Francesco Della Porta, Max-Planck-Institut für Mathematik Leipzig:
The nonlocal Cahn--Hilliard--Hele--Shaw system with regular and singular potential
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The Cahn--Hilliard--Hele--Shaw system (also called the Cahn--Hilliard--Darcy system) can be used to describe the evolution of binary immiscible fluids in a Hele--Shaw cell or in porous media, and has been recently used as a buildingblock for some tumor-growth models. In my talk, I will introduce a physically motivated nonlocal variant of the Cahn--Hilliard--Hele--Shaw system, and present some related analytical results. These include the well-posedness of the system, the existence of a global attractor, and the convergence of solutions to equilibria as time goes to infinity (in the two-dimensional case). We discuss both the case with regular and singular potential.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 12.06.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Samuel Müller, University of Sydney, Australia:
c2pLasso: The categorical-continuous pliable Lasso to identify brain regions affecting motor impairment in Huntington disease
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In many clinical studies, prediction models are essential for forecasting and monitoring the progression of a disease. Developing prediction models is a challenge when dealing with high-dimensional data since we do not know which variables are related to the response variable of interest and this relationship may depend on other continuous or categorical modifying variables as well. We formalize this problem as the varying-coefficient model selection and propose a novel variable selection method, c2pLasso, that accounts for both continuous and categorical modifying variables.
Our contributions are three-fold:
1. The c2pLasso method is shown to better screen irrelevant variables over the existing method that ignores the group structure of categorical modifying variables and to lead to a prediction model with higher accuracy and easier interpretation.
2. Our method adequately considers the pre-specified group structure among modifying variables in addition to unstructured modifying variables.
3. The c2pLasso is empirically shown to perform better than existing methods such as the Lasso and pLasso even when there is no categorical modifying variable or any pre-specified group structure among modifying variables. Using simulation studies, we show our method selects less irrelevant variables compared to existing methods while choosing relevant variables correctly. This provides us with a prediction model with higher specificity, lower false discovery rate and lower mean squared error. The proposed methodology is motivated by and illustrated using data from a Huntington disease study; the result identifies brain regions associated with motor impairment accounting for differentiated relationship by disease severity. To the best of our knowledge, our study is the first to identify the interaction effect between disease severity and the volume of brain regions in a varying-coefficient model framework.
This is joint work with Rakheon Kim and Tanya Garcia, both at Texas A&M, Department of Statistics.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
June 11, 2019 (WIAS-ESH)
Workshop/Konferenz: Colloquium in Honor of the 80th Birthday of Klaus Schneider
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Tuesday, 11.06.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Nikolas Tapia, WIAS Berlin:
Signatures in shape analysis
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Wednesday, 05.06.2019, 15:00 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Patrick Tolksdorf, Université Paris-Est - Créteil Val-de-Marne, Frankreich:
On the Lp-theory for second-order elliptic operators in divergence-form with complex coefficients (Part 2)
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Given a complex, elliptic coefficient function we investigate for which values of p the corresponding second-order divergence form operator, complemented with Dirichlet, Neumann or mixed boundary conditions, generates a strongly continuous semigroup on Lp(Ω). Additional properties like analyticity of the semigroup, H-calculus and maximal regularity are also discussed. Finally we prove a perturbation result for real coefficients that gives the whole range of p's for small imaginary parts of the coefficients. Our results are based on the recent notion of p-ellipticity, reverse Hölder inequalities and Gaussian estimates for the real coefficients.

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen" (Langenbach-Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 05.06.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Yegor Klochkov, Humboldt-Universität zu Berlin:
Influencer dynamics in opinion networks
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 04.06.2019, 13:30 (WIAS-406)
Seminar Materialmodellierung
Dr. Giselle Antunes Monteiro, Tschechische Akademie der Wissenschaften, Prag, Tschechische Republik:
On the convergence of viscous approximation for rate-independent processes with regulated inputs
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
The vanishing viscosity method is a popular tool for describing rate-independent evolution. It consists in the analysis of the limiting behavior of a regularized problem obtained by introducing a viscous dissipation mechanism which stabilizes the process. In this talk, we discuss some issues related to viscous approximations to rate-independent processes when different choices of the viscosity operator are considered. We show that the viscous limit exists, and the associated input-output operator is continuous in the space of regulated functions. Notably, we observe that the vanishing viscosity limit may exhibit some unexpected behavior when the input has some jump discontinuities.

Host
WIAS Berlin
Monday, 03.06.2019, 11:30 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Guozhi Dong, Humboldt-Universität zu Berlin:
Quantitative magnetic resonance imaging: From fingerprinting to integrated physics-based models
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this talk, we introduce a novel method for quantitative MRI. The proposed approach simultaneously recovers the tissue parameters as the recently developed method magnetic resonance fingerprinting (MRF). However, in comparison with MRF and many of its variants, our new model is dictionary free. Thus, its accuracy is independent of the discretization size of the dictionary. The efficiency of our new method is proved theoretically and also verified by numerical examples.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
WIAS Berlin
Monday, 03.06.2019, 11:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Selma Metzner, PTB / TU Berlin:
Approximate large-scale Bayesian inference with application to magnetic resonance fingerprinting
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
A class of nonlinear, large-scale regression problems is considered where the parameters model the spatial distribution of some property. Specific assumptions about the regression function are made and a homoscedastic Gaussian sampling distribution is assumed. The class of problems includes the task of Magnetic Resonance Fingerprinting (MRF), a new approach for quantitative Magnetic Resonance Imaging that allows for the estimation of absolute values of tissue related parameters like proton density and spin relaxation times. The properties of a Bayesian inference for the considered class of regression problems is investigated when different types of priors are employed, including Gaussian Markov random field priors expressing spatial smoothness. The properties of the resulting posteriors are explored and conditions for propriety of the posterior and existence of its moments established. An approximate calculation scheme is proposed and its practicability demonstrated up to dimensions of 105. Finally, the approach is applied to MRF. In using simulated data with known ground truth, it is shown that by using the prior knowledge of smoothness in the spatial distribution of the sought parameters, the results are significantly better than those achieved through maximum likelihood estimation.

Host
WIAS Berlin
Wednesday, 29.05.2019, 15:00 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Patrick Tolksdorf, Université Paris-Est - Créteil Val-de-Marne, Frankreich:
On the Lp-theory for second-order elliptic operators in divergence-form with complex coefficients (Part 1)
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Given a complex, elliptic coefficient function we investigate for which values of p the corresponding second-order divergence form operator, complemented with Dirichlet, Neumann or mixed boundary conditions, generates a strongly continuous semigroup on Lp(Ω). Additional properties like analyticity of the semigroup, H-calculus and maximal regularity are also discussed. Finally we prove a perturbation result for real coefficients that gives the whole range of p's for small imaginary parts of the coefficients. Our results are based on the recent notion of p-ellipticity, reverse Hölder inequalities and Gaussian estimates for the real coefficients.

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen" (Langenbach-Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 29.05.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Denis Belomestny, Universität Duisburg-Essen:
Density deconvolution under general assuptions on measurement error distribution
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The subject of my talk is the density deconvolution problem under general assumptions on the measurement error distribution. Typically deconvolution estimators are constructed using Fourier transform techniques, and the main assumption is that the measurement error characteristic function does not vanish on the real line. This assumption is rather strong and does not hold in many cases of interest. Here we develop a new technique to deal with this problem which allows us to recover the standard convergence rates without additional assumptions on the measurement error distribution. Joint work with A. Goldenshluger (University of Haifa).

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 28.05.2019, 15:15 (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Sebastian Eydam, WIAS Berlin:
Mode-locked solutions and coherence echoes in systems of globally-coupled phase oscillators
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In systems of Kuramoto type globally-coupled phase oscillators with equidistant natural frequencies, one can observe an interesting collective phenomenon below the synchronization threshold. The collective behavior is characterized by sharp pulses in the mean-field amplitude and therefore appropriately called mode locked. We discuss the emergence of this particular type of solution as well as the typical bifurcation scenarios that are found along their stability boundaries. In large ensembles, where the natural frequencies follow from a multi-modal distribution, mode-locked solutions are observed and the breakdown of the pulsation due to the broadening of the frequency width is explored. Echo phenomena have a long history and are found in various systems with one of the most prominent examples being the appearance of spin-echoes. We consider an echo-type response phenomenon that occurs after multiple external stimuli in ensembles of phase oscillators below the synchronization threshold. An intuitive explanation of the coherence echo phenomenon in the language of mode locking is given and it is shown that the nonlinear global interaction that supports mode-locked solutions can also enhance the echo response significantly.

Host
WIAS Berlin
Freie Universität Berlin
Tuesday, 28.05.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Paul Hager, TU Berlin:
The multiplicative chaos of fractional Brownian motions with vanishing Hurst parameters
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
Empirical estimates attest the stochastic volatility process a small path regularity, which in the context of the rough volatility models are estimates of a small Hurst parameter of the underlying fractional Brownian motion. Ever smaller estimates have motivated Neuman and Rosenbaum to investigate the limit of fractional Brownian motions with vanishing Hurst parameters. Due to the vanishing Hölder regularity, the convergence can only be meaningful in the sense of generalised functions. The authors presented a normalised sequence of fractional Brownian motions and showed the convergence towards a log-correlated field, i.e. a generalised Gaussian field with a logarithmic singularity on the diagonal of the covariance kernel. Placed in the abstract ambient of the less known fractional Gaussian fields, we find the essential property of the normalisation that makes this convergence possible and we pose a general class of normalisations, which apply to fractional Brownian fields as well. The more interesting object from the finance perspective is the volatility process, i.e the exponential of the fractional Brownian motion. The limit process is not point-wise defined, yet we can make sense of the integrated volatility process as a random measure. The theory of Gaussian multiplicative chaos deals exactly with the latter problem: Defining the exponential of singular Gaussian fields. The usual construction of these measures uses a regularisation of the field, therefore most results are not directly applicable in our situation. This problem was treated by Neuman and Rosenbaum, however their proofs were incomplete. We are able to state an elementary proof for the convergence of the volatility measures in the so call L2-phase.

Host
WIAS Berlin
Tuesday, 28.05.2019, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Björn Sprungk, Georg-August-Universität Göttingen:
Noise-level robust sampling methods for Bayesian inverse problems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Thursday, 23.05.2019, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Maria Chernysheva, Leibniz Institute of Photonic Technology, Jena:
Highlights of pulse generation and dynamics in mode-locked all-fibre and SOA-based fibre lasers
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
May 23 – 25, 2019 (WIAS-ESH)
Workshop/Konferenz: 11th Annual ERC Berlin-Oxford Young Researchers Meeting on Applied Stochastic Analysis
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Technische Universität Berlin
University of Oxford
Wednesday, 22.05.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Pierluigi Colli, Università di Pavia, Italien:
Well-posedness, regularity and asymptotic analyses for a fractional phase field system
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 22.05.2019, 11:15 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Gilian Heller, Macquarie University, Sydney:
Part II: Ordinal regression models for continuous scales
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Visual Analogue Scales (VAS) are used for measuring quantities which are intangible and difficult to measure on conventional scales, such as pain, anxiety and quality of life. These are generally used for self-rating. Subjects are given a linear scale of 100 mm and asked to put a mark where they perceive themselves. The scale has verbal anchor descriptors at each extreme, such as (in the pain context) ?no pain? and ?worst pain imaginable?. The VAS reading is taken as the measurement from the left endpoint to the subject's mark, and is usually normalized to lie in the interval [0,1]. Statistical analysis of the VAS is controversial. While it is a bounded, continuous variable, several authors have argued that it is ordinal, rather than ratio in nature, and should be treated as such. The issue is that, for example, a 1-cm difference in VAS scores at the lower end of the scale does not necessarily represent the same difference in the intangible outcome as a 1-cm difference at the upper end; and a doubling of VAS score may not translate to a doubling of e.g. the pain or anxiety. This problem is overcome by treating VAS measurements as ordinal rather than ratio data. We therefore refer to scales of this type as continuous ordinal. We have developed a regression framework for continuous ordinal responses. We express the likelihood in terms of a function connecting the scale with an underlying continuous latent variable and approximate this function non-parametrically. Then a general semi-parametric regression framework for continuous scales is developed. The model is shown to be a conditional transformation model, and is generalizable to a much wider range of uses than the context in which it was developed. We illustrate our method on a quality of life data set.

Host
Universität Potsdam
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 22.05.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Gilian Heller, Macquarie University, Sydney:
Part I: Parameter orthogonality and the GAMLSS family of distributions
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Parameter orthogonality is a desirable property of statistical distributions having more than one parameter. When parameters are orthogonal then their maximum likelihood estimates are asymptotically independent. Within the exponential family, the mean and dispersion parameter are orthogonal; however in general this is not the case. This work is motivated by a trial in Parkinson's disease patients in which one of the outcomes is the number of falls. Inspection of the data reveals that the Poisson-inverse Gaussian (PiG) distribution is appropriate, and that the experimental treatment reduces not only the mean, but also the variability, substantially. Conventional analysis assumes a treatment effect on the mean, either adjusted or unadjusted for covariates, and a constant dispersion parameter. We find that we reach quite different conclusions on the treatment effect on the mean, depending on whether or not a model is specified for the dispersion parameter.

Host
Universität Potsdam
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 21.05.2019, 15:00 (WIAS-ESH)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Fabian Telschow, University of California San Diego, USA:
Estimation of expected Euler characteristics of non-stationary Gaussian random fields
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Tuesday, 21.05.2019, 10:15 (WIAS-ESH)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Olaf Klein, WIAS Berlin:
On uncertainty quantification for models involving hysteresis effects
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
To describe hysteresis effects like magnetization or elasto-plastic behavior models involving hysteresis operators are used. The parameter within these operators are subject to uncertainties, since they have to be identified from measurements. To quantify the influence of these uncertainties, the parameter in the hysteresis operator are considered as functions of random variables. This allows to apply the methods of emphUncertainty Quantification (UQ) to the considered models. Performing UQ for models involving hysteresis effects, we get new random variables describing the uncertain outputs of the and we can compute stochastic properties of the output of the model. As example, forward UQ for a play operator with uncertain yield limits is shown. Moreover, starting from measured data for an Terfenol actuator, the identification of the parameters and their uncertainty within the hysteresis operator in the corresponding model is discussed.

Host
WIAS Berlin
Friday, 17.05.2019, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Peter Berg, University of Alberta , Kanada:
Energy conversion in electrokinetic flow through charged and viscoelastic nanochannels
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The flow of ions and water through viscoelastic, nanoscopic domains forms the basis for many processes in biological materials. Surprisingly, such systems have rarely been explored for nanofluidic transport in artificial channels of technological applications such as energy harvesting, water desalination or DNA purification.
This talk explores the nonlinear coupling between wall deformation and quasi 1-D electrokinetic transport in a nanochannel with charged walls. Within the framework of nonequilibrium thermodynamics, formulae are derived for the electrokinetic transport parameters in terms of Onsager phenomenological coefficients and, subsequently, for energy conversion efficiencies. Results confirm that Onsager's reciprocity principle holds for rigid channels but breaks down in the 1-D formulation when the channel is deformed due to the introduction of a ”fictitious” diffusion term of counter-ions. Furthermore, the model predicts a reduced efficiency of electrokinetic energy harvesting for channels with soft, deformable walls.
This research is conducted in collaboration with Michael Eikerling and Mpumelelo Matse.

Host
WIAS Berlin
Thursday, 16.05.2019, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Robert Eisenberg, Rush University Chicago, USA:
Voltage sensors of biological channels are nanomachines that perfectly conserve current, as Maxwell defined it
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Biological channels produce the signals of the nervous system, and coordinate the contraction of muscle, including the heart, by responding to voltage. Biological channels are proteins with a specific piece of machinery that responds to voltage, called the voltage sensor. The voltage sensor moves charges through an electric field creating a polarization (i.e., dielectric) current that can be measured in the far field some 1e23 atoms away from the channel because Maxwell's equations enforce the perfect conservation of current, as Maxwell defined it. Maxwell's current includes the polarization of the vacuum, independent of the properties of matter (including proteins!) no matter how complex. We have built a precise electromechanical model of the voltage sensor based on its atomic scale structure that fits a wide range of experimental data.

Host
WIAS Berlin
Wednesday, 15.05.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Robert Eisenberg, Rush University Chicago:
Ionic selectivity in channels: complex biology created by the balance of simple physical forces
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 15.05.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Frank Konietschke, Charité Berlin:
Small data: A big data problem
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Small sample sizes occur frequently and especially in preclinical research. Most statistical methods are only valid if sample sizes are large and thus, investigating the methods' behavior when samples are small is tempting. It turns out, that few statistical methods are as reliable as throwing a coin when samples are small. In this talk, we propose few improvements using resampling and permutation methods. In particular, we will answer the question "When and how do permutation methods work?". Real data sets illustrates the application of the proposed mean based and purely nonparametric rank-based methods.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 14.05.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Michele Coghi, WIAS Berlin:
Rough nonlinear Fokker-Planck equations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
Motivated by McKean-Vlasov diffusions with "common" noise, we present well-posedness of a nonlinear Fokker-Planck equation driven by a rough path. Joint work with Torstein Nilssen.

Host
WIAS Berlin
Tuesday, 14.05.2019, 13:30 (WIAS-406)
Seminar Materialmodellierung
Dr. Mirjam Walloth, Technische Universiät Darmstadt:
Reliable, efficient and robust a posteriori estimators for the variational inequality in fracture phase-field models
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Monday, 13.05.2019, 14:00 (WIAS-ESH)
Seminar Quantitative Biomedizin
Prof. Robert Eisenberg, Rush University Chicago, USA:
The lens of the eye: Bidomain model of an osmotic pump
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The lens of the eye has no blood vessels to interfere with vision. The lens is far too large for diffusion to provide food and clear wastes. Experimental, theoretical and computational work has shown that the lens supports its own microcirculation. It is an osmotic pump. We introduce a general non-electro-neutral model that describes the steady state relationships among ion fluxes, water flow and electric field inside cells, and in the narrow extracellular spaces within the lens. Using asymptotic analysis, we derive a simplified model based on physiological data. The model reduces to the first generation analytical models. The full model explains additional data of some significance.

Host
WIAS Berlin
Thursday, 09.05.2019, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Sebastian Franke, Technische Universität Berlin:
Quantization of dissipative resonators using quasinormal modes
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Thursday, 09.05.2019, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Christoph Freysoldt, Max-Planck-Institut für Eisenforschung GmbH:
Concepts and algorithms in SPHInX
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
SPHInX is a C++ class library for computer simulations in material science, with a focus on electronic-structure theory. Our aim is to provide a framework that is on one hand enables physicists to implement easily new algorithms, and on the other hand makes use of developments in computer hardware and software design principles to achieve efficiency. I will give an overview over the hierarchy of available classes, and exemplify how mathematical and physical concepts are mapped onto C++ classes. I will then demonstrate the mutual benefit of concept and algorithm evolution. Last, some seemingly minor features of the SPHInX package will be highlighted, such as debugging checks, clocks, file parsing, or command line options, that proved to be immensely productive in daily work.

Host
WIAS Berlin
Wednesday, 08.05.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Amal Alphonse, WIAS Berlin:
Well posedness for a coupled bulk-surface system on a moving domain modelling ligand-receptor dynamics
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
This talk is on the well posedness and regularity of solutions of a reaction-diffusion system of coupled bulk-surface equations on a moving domain modelling receptor-ligand dynamics in cells. The coupling of the unknowns is through a nonlinear Robin boundary condition for the bulk quantity and the source terms for the two surface quantities. After motivating the model, I will discuss the difficulties posed by this system and the technical tools necessary in order to achieve the desired analytical results. This talk is based on a recent joint paper with Charlie Elliott (University of Warwick, UK) and Joana Terra (Universidad Nacional de Cordoba, Argentina).

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 07.05.2019, 13:30 (WIAS-406)
Seminar Materialmodellierung
Dr.-Ing. Rainer Falkenberg, Bundesanstalt für Materialforschung und -prüfung:
Aspects on the modelling of material degradation
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
Material degradation describes the loss of nominal strength. The physical causes as well as the consequences are often manifold: Mechanical loads exceeding a threshold value or temperature/species-induced effects are possible and can lead e.g. to a reduced load-bearing capacity in general or to crack initiation and propagation in a local sense. The formulation and solution of this initial boundary value problem must therefore cover some crucial aspects: e.g. the fulfillment of the second law of thermodynamics by the constitutive as well as the degradation model or the consideration of the PDE system's stability loss when dealing with strict local models. Well-established models that will be discussed in the finite-element framework are the fracture-mechanics based cohesive zone model, the damage-mechanics based phase-field model and the micromechanics-based Gurson--model. Furthermore, an extension will be presented that allows for the simulation of corrosion processes.

Host
WIAS Berlin
Tuesday, 30.04.2019, 15:15 (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Irina Kmit, Humboldt-Universität zu Berlin:
Classical bounded and almost periodic solutions to quasilinear first-order hyperbolic systems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We consider boundary value problems for quasilinear first-order one-dimensional hyperbolic systems in a strip. The boundary conditions are supposed to be of a smoothing type, in the sense that the L2-generalized solutions to the initial-boundary value problems become eventually C2-smooth for any initial L2-data. We investigate small global classical solutions and obtain the existence and uniqueness result under the condition that the evolution family generated by the linearized problem has exponential dichotomy on R. We prove that the dichotomy survives under small perturbations in the leading coefficients of the hyperbolic system. Assuming that the coefficients of the hyperbolic system are almost periodic, we prove that the bounded solution is almost periodic also.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 30.04.2019, 15:00 (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Melina Freitag, University of Bath, UK:
A low-rank approach to the solution of weak constraint variational data assimilation problems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Host
WIAS Berlin
Tuesday, 30.04.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Prof. Deb Kane, Macquarie University Sydney, Australia:
Experimental nonlinear semiconductor-laser-based systems: Rich resources for applications and nonlinear science
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Nonlinear laser systems provide rich opportunities for both theoretical modelling and for technological applications. There is significant diversity in the dynamics of the output power as a function of time, that can be derived from a single nonlinear laser system for different operating conditions, or, across different nonlinear laser systems. We have been undertaking experiments that generate high density, and/or long time series, datasets from various nonlinear laser systems as a resource for facilitating comparisons between theory and experiment, and for answering science questions. Some of our recent work will be discussed including (i) dynamical mapping of semiconductor lasers with delayed optical feedback systems, both integrated multi-section systems and those using free space propagation; (ii) the dynamical diversity of the output of a long cavity VECSEL system primarily targeted for low pulse repetition rate passive mode-locking; and (iii) forecasting the impact of partial coherence of delayed feedback light in systems.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 24.04.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Dirk Peschka, WIAS Berlin:
Modeling, simulation, and experiments for flows of concentrated suspensions
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 24.04.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Chen Huang, Universität St. Gallen, Schweiz:
LASSO in time and space
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We consider the estimation and inference in a system of high-dimensional regression equations allowing for temporal and cross-sectional dependency in covariates and error processes, covering rather general forms of weak dependence. A sequence of regressions with many regressors using LASSO (Least Absolute Shrinkage and Selection Operator) is applied for variable selection purpose, and an overall penalty level is carefully chosen by a block multiplier bootstrap procedure to account for multiplicity of the equations and dependencies in the data. Correspondingly, oracle properties with a jointly selected tuning parameter are derived. We further provide high-quality de-biased simultaneous inference on the many target parameters of the system. We provide bootstrap consistency results of the test procedure, which are based on a general Bahadur representation for the Z-estimators with dependent data. Simulations demonstrate good performance of the proposed inference procedure. Finally, we apply the method to quantify spillover effects of textual sentiment indices in a financial market and to test the connectedness among sectors.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 23.04.2019, 13:30 (WIAS-406)
Seminar Materialmodellierung
Marijo Milicevic, Albert-Ludwigs-Universität Freiburg:
The alternating direction method of multipliers with variable step sizes for the iterative solution of nonsmooth minimization problems and application to BV-damage evolution
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
The alternating direction method of multipliers (ADMM) is a flexible numerical method to solve a large class of convex minimization problems. Its most significant properties are the unconditional convergence with respect to the involved step size and the direct applicability. However, the performance critically depends on the choice of the step size. We propose an automated step size adjustment that relies on the monotonicity of the residual to accelerate the ADMM. Numerical experiments show a remarkable improvement over the standard ADMM with fixed step sizes. The ADMM with variable step sizes is then applied to a model for rate-independent, total variation regularized damage processes. The total variation regularization of the damage variable leads to sharp transitions of damaged to undamaged areas in the material. The results are compared to an H1-regularization of the damage and the simulations reveal that, indeed, for the total variation regularization sharp transitions can be observed whereas for the H1-regularization the interface is smeared out.

Host
WIAS Berlin
Wednesday, 17.04.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Grigor Nika, WIAS Berlin:
Multiscale modelling of magnetorheological suspensions
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 16.04.2019, 15:15 (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Dr. Oleh Omelchenko, Universität Potsdam:
Traveling chimera states
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We overview recent results about the traveling chimera states observed in a ring of nonlocally coupled phase oscillators with broken reflection symmetry of the coupling kernel. These states manifest themselves as coherence-incoherence patterns moving along the ring. As the coupling asymmetry grows they undergo a sequence of transformations, which can be adequately explained using the continuum limit Ott-Antonsen equation. In the context of this equation the chimera states correspond to its smooth traveling wave solutions. Using the mathematical methods from the PDE and integral equations theory we carry out asymptotic analysis of these traveling waves, describe an algorithm for their numerical continuation and explore the spectrum of the corresponding linearized equation. We show that traveling chimera states can lose their stability via fold and Hopf bifurcations. Some of the Hopf bifurcations turn out to be supercritical resulting in the observation of modulated traveling chimera states.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 16.04.2019, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Jérôme Lelong, Université Grenoble Alpes, Frankreich:
Pricing path-dependent Bermudan options using Wiener chaos expansion: An embarrassingly parallel approach
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
In this work, we propose a new policy iteration algorithm for pricing Bermudan options when the payoff process cannot be written as a function of a lifted Markov process. Our approach is based on a modification of the well-known Longstaff Schwartz algorithm, in which we basically replace the standard least square regression by a Wiener chaos expansion. Not only does it allow us to deal with a non Markovian setting, but it also breaks the bottleneck induced by the least square regression as the coefficients of the chaos expansion are given by scalar products on the L^2 space and can therefore be approximated by independent Monte Carlo computations. This key feature enables us to provide an embarrassingly parallel algorithm.

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Tuesday, 16.04.2019, 13:30 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Li Jie, University of Cambridge, GB:
Macroscopic model for head-on binary droplet collisions in a gaseous medium
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this work, coalescence-bouncing transitions of head-on binary droplet collisions are predicted by a novel macroscopic model based entirely on fundamental laws of physics. By making use of an existing lubrication theory, we have modified the Navier-Stokes equations to accurately account for the rarefied nature of the interdroplet gas film. Through the disjoint pressure model, we have incorporated the intermolecular Van der Waals forces. Our model does not use any adjustable (empirical) parameters. It therefore encompasses an extreme range of length scales (more than 5 orders of magnitude): from those of the external flow in excess of the droplet size (a few hundred micros) to the effective range of the Van der Waals force around 10 nm. A state of the art moving adaptive mesh method, capable of resolving all the relevant length scales, has been employed. Our numerical simulations are able to capture the coalescence-bouncing and bouncing-coalescence transitions that are observed as the collision intensity increases. The predicted transition Weber numbers for tetradecane and water droplet collisions at different pressures show remarkably good agreement with published experimental values. Our study also sheds new light on the roles of gas density, droplet size and mean free path in the rupture of the gas film.

Host
WIAS Berlin
Thursday, 28.03.2019, 16:00 (WIAS-ESH)
Seminar Laserdynamik
Christian Schelte, University of the Balearic Islands, Spain:
Bistability and solitons in an injected time-delayed Kerr microcavity
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Seminar Laserdynamik

Host
WIAS Berlin
Thursday, 28.03.2019, 10:00 (WIAS-406)
Seminar Laserdynamik
Aida Hajizadeh, Leibniz-Institut für Neurobiologie Magdeburg:
Computational modelling of signal processing in the human auditory cortex: Analytical solutions based on cortical network structure and oscillatory dynamics
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Further Informations
Seminar Laserdynamik

Host
WIAS Berlin
Wednesday, 27.03.2019, 10:30 (WIAS-406)
Seminar Interacting Random Systems
Dr. Giovanni Luca Torrisi, Istituto per le Applicazioni del Calcolo “Mauro Picone” (C.N.R), Italien:
The Clark-Ocone formula for point processes
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
Clark-Ocone formulas are powerful results in stochastic analysis with a variety of applications. In the talk we provide the Clark-Ocone formula for square-integrable functionals of point processes with stochastic intensity. Then we present two applications of the formula: the Poincare' inequality and a deviation bound for those functionals. Our results generalize the corresponding ones on the Poisson space

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Tuesday, 26.03.2019, 15:00 (WIAS-ESH)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Valeriy Avanesov, WIAS Berlin:
Nonparametric change point detection in regression
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
March 20 – 22, 2019 (IAP)
Workshop/Konferenz: Leibniz MMS Days 2019
more ... Location
Leibniz-Institute of Atmospheric Physics, Schlossstr. 6, 04318 Kühlungsborn, Germany

Host
WIAS Berlin
Tuesday, 19.03.2019, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Darina Dvinskikh, WIAS Berlin:
Distributed decentralized (stochastic) optimization for dual friendly functions
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We study the optimal complexity bounds of first-order methods for the problem of minimizing the finite sum of convex (strongly convex) functions. To address this problem, we appeal to distributed optimization and consider the network of multiple agents (processors) which cooperatively minimize the target function. We compare two different approaches toward the finding of the minimum depending on the functions properties in terms of oracle calls and communication rounds: primal approach and dual approach, based on the assumption of existence of the Fenchel-Legendre transform of the target function.

Host
WIAS Berlin
March 18 – 22, 2019 (ZIB-SEM)
Workshop/Konferenz: New Directions in Stochastic Analysis: Rough Paths, SPDEs and Related Topics
more ... Location
Zuse-Zentrum für Informationstechnik (ZIB), Takustr. 7, 14195 Berlin, Seminarraum

Host
Technische Universität Berlin
WIAS Berlin
Universität Potsdam
Freie Universität Berlin
DFG Research Unit 2402
European Research Council
Tuesday, 12.03.2019, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Malte Weber, Humboldt-Universität zu Berlin:
Community detection using belief propagation
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
Using the message passing algorithm "belief propagation" to detect communities in a network graph, i.e. finding subsets of the nodes of the graph which are more densly connected within, than they are to the other nodes of the graph.

Host
WIAS Berlin
Thursday, 28.02.2019, 10:00 (WIAS-406)
Seminar Materialmodellierung
Prof. Uwe Thiele, Westfälische Wilhelms-Universität Münster:
Gradient dynamics models for films of complex fluids and beyond - dewetting, line deposition and biofilms
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
After briefly reviewing a number of experiments on dewetting and evaporating thin films/drops of simple and complex liquids, I introduce the concept of a gradient dynamics description of the evolution of interface-dominated films and drops on solid substrates. First, the case of films/drops of simple non-volatile liquid is discussed, and illustrated with results on droplet patterns and sliding droplets. As a further example, the diffusion equation is formulated as a gradient dynamics. The obtained elements are combined into a thermodynamically consistent gradient dynamics formulation for films of mixtures and surfactant suspensions [1,2]. Next, such models are employed to investigate the out-of-equilibrium process of the deposition of line patterns at receding contact lines for evaporatively dewetting solutions/suspensions [3] and in Langmuir Blodgett transfer [4]. Finally, I discuss how to combine the presented thin-film dynamics with bioactive elements to obtain models for the osmotic spreading of biofilms growing on moist agars [5]. I conclude with a summary and outlook. [1] U. Thiele, D. Todorova, H. Lopez, Phys. Rev. Lett. 111, 117801 (2013) [2] Thiele, U.; Archer, A.Pismen, L., Phys. Rev. Fluids 1, 083903 (2016). [3] U. Thiele, Advances in Colloid and Interface Science 206, 399-413 (2014). [4] M.H. Köpf and U. Thiele, Nonlinearity 27, 2711-2734 (2014). [5] Trinschek, S.; John, K.; Lecuyer, S., Thiele, U., Phys. Rev. Lett. 119, 078003 (2017). [6] Trinschek, S.; John, K. Thiele, U., Soft Matter 14, 4464-4476 (2018)

Host
WIAS Berlin
Tuesday, 26.02.2019, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr Paolo Pigato, WIAS Berlin:
Density estimates for stocastic differential equations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Tuesday, 19.02.2019, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Jan Schwientek; Holger Berthold, Fraunhofer-Institut für Techno- und Wirtschaftsmathematik ITWM:
Numerical methods for (general) semi-infinite optimization
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
Semi-infinite programming has become a powerful modeling tool in (continuous) optimization tasks and is increasingly used in many practical problems from various fields recently. This is mainly due to the enormous further development of computer technology and numerical methods, which now make such problems tractable on standard PCs within reasonable time. The talk is divided into two parts. Firstly, we give a short introduction to (general) semi-infinite programming and survey its wide application range. We then review numerical methods from literature and present new promising approaches from ITWM. In the second part of the talk we give a detailed numerical comparison of several methods on basis of an exhaustive set of test problem, partly stemming from real-world applications. The focus of this comparison is on the performance of an ITWM solver opposing the other introduced numerical methods concerning running time, needed iterations, and accuracy. In doing so we investigate the behaviour of the implemented solvers also with respect to different mathematical environments like convexity and dimensionality.

Host
WIAS Berlin
Thursday, 14.02.2019, 14:30 (WIAS-ESH)
Seminar Numerische Mathematik
Philipp Schroeder, Georg-August-Universität Göttingen:
Building bridges: Pressure-robust FEM, Beltrami flows and structure preservation in incompressible CFD
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this talk, an attempt is made to explain why pressure-robust FEM are (by construction) superior when it comes to simulating incompressible flows with a large amount of large-scale/coherent structures. We show that flows with large-scale structures are frequently dominated by large gradient (curl-free) forces. Pressure-robust methods are designed in exactly such a way that they can treat those forces more accurately than non-pressure-robust methods. Furthermore, the class of generalised Beltrami flows is introduced and placed in context with structure preservation and pressure-robustness. Several numerical examples, both in 2D and in 3D, of laminar and turbulent flows, are shown which underline our statements. All pressure-robust computations make us of (high-order) exactly divergence-free H(div)-(H)DG methods.

Host
WIAS Berlin
Wednesday, 13.02.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Alexander Mielke, WIAS Berlin:
On similarity solutions for Kolmogorov's two-equation model for turbulence
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 13.02.2019, 11:30 (WIAS-406)
Seminar Interacting Random Systems
Peter Neijjar, Institute of Science and Technology Austria (IST Austria), Österreich:
Product limit laws at shocks in (T)ASEP
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We consider the asymmetric simple exclusion process (ASEP) on Z with an initial data such that in the large time particle density r a discontinuity at the origin is created, where the value of r jumps from zero to one, but r (resp. 1-r) is strictly positive to the left (resp. right) of the origin. We consider the position of a particle macroscopically located at the discontinuity, and show that its limit law has a cutoff at the origin. Inside the discontinuity region, we show that a discrete product limit law arises, which bounds from above the limiting fluctuations of the particle in the general ASEP, and equals them in the totally ASEP. Sending the size of the shock region to infinity, we recover the product of GUE distributions previously observed at shocks in the totally ASEP.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Wednesday, 13.02.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Professor Albert Cohen, Université Pierre et Marie Curie, Paris, France:
Optimal non-intrusive methods in high-dimension
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Motivated by non-intrusive approaches for high-dimensional parametric PDEs, we consider the approximation of an unknown arbirary function in any dimension from the data of point samples, where the approximants are picked from given or adaptively chosen finite dimensional spaces. One principal objective is to obtain an approximation which performs as good as the orthogonal projection using a sampling budget that is linear in the dimension of the approximating space. Using a particular sampling measure, this objective turns out to be met by both least-squares and pseudo-spectral methods in some probabilistic sense, however with some notable distinctions that will be discussed in this talk.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Thursday, 07.02.2019, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Prof. Raimondas Čiegis, Vilnius Gediminas Technical University, Lithuania:
Numerical simulation of models with the memory effects: Diffusion, heat conduction, nonlinear optics
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Wednesday, 06.02.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Serhiy Yanchuk, Technische Universität Berlin:
Temporal dissipative solitons in delay feedback systems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 06.02.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Christian Clason, Universität Duisburg-Essen:
Generalized modes in Bayesian inverse problems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this talk, we consider non-parametric modes and MAP estimates for priors that do not admit continuous densities, for which previous approaches based on small ball probabilities fail. We propose a novel de nition of generalized modes based on the concept of approximating sequences, which reduce to the classical mode in certain situations that include Gaussian priors but also exist for a more general class of priors. The latter includes the case of priors that impose strict bounds on the admissible parameters and in particular of uniform priors. For uniform priors de ned by random series with uniformly distributed coecients, we show that generalized MAP estimates but not classical MAP estimates can be characterized as minimizers of a suitable functional that plays the role of a generalized OnsagerMachlup functional. This is then used to show consistency of nonlinear Bayesian inverse problems with uniform priors and Gaussian noise. We also address the numerical computation of the corresponding generalized MAP estimates.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 05.02.2019, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Jórg Polzehl, WIAS Berlin:
Towards in-vivo histology of the brain - some statistical contributions
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Wednesday, 30.01.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Barbara Zwicknagl, Technische Universität Berlin:
On variational models for (almost) stress-free martensitic inclusions
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 30.01.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Jean-Pierre Florens, University of Toulouse:
Is completeness necessary? Penalized estimation in non identified linear models
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 29.01.2019, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Alejandro Kocsard, Universidade Federal Fluminense, Brazil:
Random walks, synchronization and existence of invariant measures
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstract
In this talk we start discussing some classical results due to Furstenberg [Fur63] about random product of matrices and how a certain form of synchronization naturally appears in this context. Then we shall consider some extensions of these results for random walks on compact smooth manifolds [Led84, Ant84, Cra90]. Finally we will see how these ideas can be used to study the existence of invariant (probability) measures for smooth actions on manifolds by some groups which are a priori non-amenable.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 29.01.2019, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Darina Dvinskikh, WIAS Berlin:
Distributed decentralized (stochastic) optimization for dual-friendly functions
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Tuesday, 29.01.2019, 13:30 (WIAS-406)
Seminar Materialmodellierung
Prof. Vittorio Romano, University of Catania, Italy:
Charge and phonon transport in graphene
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
The last years have witnessed a great interest for 2D-materials due to their promising applications. The most investigated one is graphene which is considered as a potential new material to exploit in nano-electronic and optoelectronic devices. Charge transport in graphene can be described with several degrees of physical complexity. At quantum level an accurate model is represented by the Wigner equation but in several cases its semiclassical limit, the Boltzmann equation, constitutes a fully acceptable model. However, the numerical difficulties encountered in the direct solution of both the Wigner and the semiclassical Boltzmann equation has prompted the development of hydrodynamical, energy transport and drift diffusion models, in view of the design of a future generation of electron devices where graphene replaces standard semiconductors like silicon and gallium arsenide. Moreover, thermal effects in low dimensional structures play a relevant role and, therefore, also phonon transport must be included. Interesting new mathematical issues related to the peculiar features of graphene arise. The main aspects will be discussed and recent results illustrated in the perspective of future developments, in particular the optimization of graphene field effect transistors.

Host
WIAS Berlin
Thursday, 24.01.2019, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Stefan Schulz, Tyndall National Institute, Ireland:
Electronic, optical and transport properties of III--N alloys and heterostructures: Insights from and challenges for theoretical modelling
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
InGaN-based heterostructures have attracted considerable interest for a variety of different applications. For example, the key building blocks of modern light-emitting devices operating in the blue spectral region are c-plane InGaN/GaN quantum wells (QWs). It is remarkable that these devices are so successful, given the extremely high defect densities in InGaN/GaN systems. This defect insensitivity has been widely attributed to carrier localization effects introduced by alloy fluctuations, preventing carriers to reach non-radiative recombination centers. It is only recently that localization effects in InGaN QWs have been addressed in theoretical studies and, in comparison to other III-V materials and alloys, such such as InGaAs, several key properties are not well understood. This lack of understanding ranges from fundamental material properties up to devices related aspects. In this talk, electronic and optical properties of bulk alloys, such as AlInN or InGaN, as well as InGaN quantum wells will be discussed by means of atomistic models. These approaches range from first-principles density functional methods up to in-house developed empirical tight-binding models. Here, special attention is paid to nanoscale effects such as local fluctuations in intrinsic built-in fields. Theoretical results will be compared to available experimental data. Finally, theoretical and modeling challenges for instance in the area of transport calculations will be discussed and reviewed. Initial results of coupled tight-binding non-equilibrium Green?s function transport calculations will be presented.

Host
WIAS Berlin
Wednesday, 23.01.2019, 15:00 (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Vladimir A. Garanzha, Russian Academy of Sciences:
Moving adaptive meshes based on the hyperelastic stress deformation
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We suggest an algorithm for the time-dependent mesh deformation based on the minimization of hyperelastic quasi-isometric functional without introducing time derivatives. The source of deformation is a time-dependent metric tensor in Eulerian coordinates. Due to the nonlinearity of the Euler-Lagrange equations we can not assume that the norm of its residual is reduced to zero at each time step. Thus, time continuity of the moving mesh is not guaranteed since iterative minimization may result in considerable displacements even for infinitely small time steps. To solve this problem, we introduce a special variant of factorized representation of Lagrangian metric tensor and nonlinear interpolation procedure for factors of this metric tensor. Continuation problem with respect to interpolation parameters is similar to the hypoelastic deformation with a controlled stress relaxation. At each time step we start with a Lagrangian metric tensor which completely eliminates internal elastic stresses and makes current mesh an exact solution of the Euler-Lagrange equations. The continuation procedure gradually introduces internal stresses back while forcing the deformation to follow the prescribed Eulerian metric tensor. At each step of the continuation procedure the functional is approximately minimized using a few steps of the parallel preconditioned gradient search algorithm. We derive an auxiliary discrete evolution equation for target shape matrices (factors of Lagrangian metric tensor) which resembles stress relaxation equations in hypoelasticity. We present 2d and 3d examples of moving deforming meshes which serve to represent moving bodies for parallel immersed boundary flow solver.

Host
WIAS Berlin
Wednesday, 23.01.2019, 13:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Prof. Dr. Maria Soledad Aronna, Technische Universität:
On second order optimality conditions for control-affine problems: the finite and infinite dimensional case
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this talk I will present the main features of first and second order optimality conditions for optimal control problems of ordinary differential equations that are affine with respect to the control. Problems will be considered in the presence of control constraints. Both necessary and sufficient conditions for optimality will be presented. I will then show the extension of those results for a class of bilinear partial differential equations. Finally, if time allows it, I will briefly discuss the state-constrained case.

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 23.01.2019, 11:30 (WIAS-406)
Seminar Interacting Random Systems
Pierre Houdebert, Aix-Marseille Université, Frankreich:
Sharp phase transition for the Widom-Rowlinson model
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
The Widom-Rowlinson model is formally defined as two homogeneous Poisson point processes forbidding the points of different type to be too close. For this Gibbs model the question of uniqueness/ non-uniqueness depending on the two intensities is relevant. This model is famous because it was the first continuum Gibbs model for which phase transition was proven, in the symmetric case of equal intensities large enough. But nothing was known in the non-symmetric case, where it is conjectured that uniqueness would hold. In a recent work with D. Dereudre (Lille), we partially solved this conjecture, proving that for large enough activities the phase transition is only possible in the symmetric case of equal intensities. The proof uses percolation and stochastic domination arguments.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Tuesday, 22.01.2019, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Edgard Pimentel, Pontifical Catholic University of Rio de Janeiro, Brazil:
Geometric methods in regularity theory for nonlinear PDEs
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstract
In this talk we examine the regularity theory of the solutions to a few examples of nonlinear PDEs. Arguing through a genuinely geometrical method, we produce regularity results in Sobolev and Hölder spaces, including some borderline cases. Our techniques relate a problem of interest to a further one - for which a richer theory is available - by means of a geometric structure, e..g., a path. Ideally, information is transported along such a path, giving access to finer properties of the original equation. In the first part of the talk, we cover examples including elliptic and parabolic fully nonlinear problems, the Isaacs equation and a double divergence model. Then we proceed to the setting of state-dependent degenerate problems and report recent (optimal) results. We close the talk with a discussion on open problems and further directions of work.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 22.01.2019, 15:00 (WIAS-ESH)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Ahmed Kebaier, Université Paris 13, France:
Asymptotic properties of maximum likelihood estimator for the drift parameters of jump-type square root processes
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Wednesday, 16.01.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Joachim Rehberg, WIAS Berlin:
An extrapolation for the Lax-Milgram isomorphism for second order divergence operators
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In 1989 Gröger generalized the famous isomorphy result of Meyer to a setting which allows for mixed boundary conditions. We again generalize Gröger's result to much more general geometric configurations.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 16.01.2019, 13:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Tobias Keil, WIAS Berlin:
Optimal control of a coupled Cahn--Hilliard--Navier--Stokes system with variable fluid densities
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
This talk is concerned with the optimal control of two immiscible fluids with non-matched densities. For the mathematical formulation of the fluid phases, we use a coupled Cahn-Hilliard/Navier-Stokes system by Abels, Garcke and Grün, which involves a variational inequality of fourth order. We verify the existence of solutions to a suitable time discretization of the system and formulate an associated optimal control problem. We further discuss the differentiability properties of the control-to-state operator and the corresponding stationarity concepts and present strong stationarity conditions for the problem. This enables us to provide a numerical solution algorithm which terminates at an at least C-stationary point which - in the best case - is even strongly stationary. The method is based on an adaptation of a bundle-free implicit programming approach for MPECs in function space presented by Hintermüller and Surowiec in 2016.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
WIAS Berlin
Wednesday, 16.01.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Mathias Trabs, Universität Hamburg:
Parameter estimation for stochastic PDEs based on discrete observations in time and space
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Motivated by random phenomena in natural science as well as by mathematical finance, stochastic partial differential equations (SPDEs) have been intensively studied during the last fifty years with a main focus on theoretical analytic and probabilistic aspects. Thanks to the exploding number of available data and the fast progress in information technology, SPDE models become nowadays increasingly popular for practitioners, for instance, to model neuronal systems or interest rate fluctuations to give only two examples. Consequently, statistical methods are required to calibrate this class of complex models. We study the parameter estimation for parabolic, linear, second order SPDEs observing a mild solution on a discrete grid in time and space. Focusing first on volatility estimation and assuming a high-frequency regime in time, we provide an explicit and easy to implement method of moments estimator based on squared time increments of the process. If the observation frequency in time is finer than in space, the estimator is consistent and admits a central limit theorem. This is established moreover for the estimation of the integrated volatility in a semi-parametric framework. In a second step, we consider not only time increments of the solution field but also space increments as well as space-time increments. This allows for the construction of estimators which are robust with respect to the sampling regime, i.e., they are also applicable if the observation grid in space is finer than in time. Finally, we discuss the estimation of the parameters in the differential operator which determines the SPDE. This talk is based on joint works with Markus Bibinger and Florian Hildebrandt.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 15.01.2019, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Prof. Willie Hsia, National Taiwan University:
On the mathematical analysis of the synchronization theory with time-delay effect
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstract
In this lecture, we will introduce a newly developed mathematical theory on the synchronized collective behavior of the Kuramoto oscillators with time-delayed interactions and phase lag effect. Both the phase synchronization and frequency synchronization are in view. This is joint work with Bongsuk Kwon, Chang-Yeol Jung and Yoshihiro Ueda.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 15.01.2019, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexander Gasnikov, Moscow Institute of Physics and Technology, Russische Föderation:
Adaptive accelerated stochastic gradient descent
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Friday, 11.01.2019, 15:00 (WIAS-406)
Seminar Interacting Random Systems
Dr. Darcy Camargo, The Weizmann Institute of Science, Israel:
The meteor process stationary distribution
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
The Potlatch and Smoothing processes have a long story of studies and application, but even today not much is known about their stationary distributions when they exist. In this talk we will present the simplest version of such models and expose recent developments and techniques used to get them, especially the coupling by WIMPs and Matching.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Thursday, 10.01.2019, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Naveed Ahmed, Lahore University of Management Sciences, Pakistan:
Numerical comparisons of finite element stabilized methods for a 2D vortex dynamics simulation at high Reynolds number
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this talk, I will present an up-to-date and classical Finite Element (FE) stabiliz ed methods for time-dependent incompressible flows. All studied methods belong to the Variational MultiScale (VMS) framework. So, different realizations of stabilized FE-VMS methods are compared in a high Reynolds number vortex dynamics simulation. In particular, a fully Residual-Based (RB)-VMS method is compared with the classical Streamline-Upwind Petrov--Galerkin (SUPG) method together with grad-div stabilization, a standard one-level Local Projection Stabilization (LPS) method, and a recently proposed LPS method by interpolation. These procedures do not make use of the statistical theory of equilibrium turbulence, and no ad-hoc eddy viscosity modeling is required for all methods. Applications to the simulation of a high Reynolds numbers flow with vortical structures on relatively coarse grids are showcased, by focusing on a two-dimensional plane mixing-layer flow. Both Inf-Sup Stable (ISS) and Equal Order (EO) (H^1)-conforming FE pairs are explored, using a second-order semi-implicit Backward Differentiation Formula (BDF2) in time. Based on the numerical studies, it is concluded that the SUPG method using EO FE pairs performs best among all methods. Furthermore, there seems to be no reason to extend the SUPG method by the higher order terms of the RB-VMS method.

Host
WIAS Berlin
Wednesday, 09.01.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar) & Seminar Materialmodellierung
Prof. Dr. Peter Friz, TU Berlin/ WIAS Berlin:
On differential equations with singular forcing
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
There is much recent progress in our ability to analyze (ordinary and partial) differential equations subjected to singular forcing. This will not be a big survey talk, I will instead focus on some explicit examples.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 09.01.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Frank Werner, Georg-August-Universität Göttingen:
Empirical risk minimization as parameter choice rule for filter-based regularization methods
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We consider a-posteriori parameter choice rules for filter based linear regularization methods in the statistical inverse problem setting. In particular, we investigate the choice of the regularization parameter by minimizing an unbiased estimate of the predictive risk. This parameter choice rule and its usage are well-known in the literature, but oracle inequalities and optimality results in this general setting are unknown. We prove a (generalized) oracle inequality, which relates the direct risk with the minimal prediction risk. From this oracle inequality, we are then able to conclude that filter based regularization methods with the investigated parameter choice rule achieve convergence rates of optimal order with respect to the mean integrated squared error in mildly ill-posed problems. Finally, we also present numerical simulations, which support the order optimality of the method and the finite sample performance of the parameter choice. In these simulations, we also investigate the behavior of different a posteriori parameter choice methods in exponentially ill-posed problems. This is joint work with Housen Li (University of Göttingen).

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Wednesday, 19.12.2018, 11:30 (WIAS-406)
Seminar Interacting Random Systems
Carina Betken, Universität Osnabrück:
Dynamic random graphs: Sedentary random waypoint and preferential attachment models
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Wednesday, 19.12.2018, 11:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Olivier Huber, Humboldt-Universität zu Berlin:
Using reformulations in nonsmooth mathematical programming: the examples of friction contact problems and optimal value functions
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this talk we illustrate how reformulations can be used to deal with nonsmoothness. In particular, we look at two specific structures arising from applications: the first one concerns finite dimensional variational inequalities with a nonsmooth functional. This is motivated by the study of contact problems with Coulomb friction. The second example involves optimization models with Optimal Value Functions (OVF) in the problem data. The OVF concept subsumes regularizers from fitting problems and coherent risk measures from Stochastic Optimization. Each reformulation scheme rely on tools from convex and variational analysis. In both instances, we obtain equivalent problems that are solvable by off-the-shelf solvers. Finally, numerical results are presented and discussed: in the OVF case, the example is a risk-averse equilibrium problem from the electricity market.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 12.12.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Joachim Rehberg, WIAS Berlin:
L bounds for the Neumann problem
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 12.12.2018, 11:30 (WIAS-406)
Seminar Interacting Random Systems
Prof. Dr. Kwabena Doku-Amponsah, University of Ghana School of Physical and Mathematical Sciences, Ghana, Republik:
Joint large deviation result for empirical measures of the random geometric graphs
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We prove joint large deviation principle for the empirical pair measure and empirical locality measure of the near intermediate coloured random geometric graph models on n points picked uniformly in the space [0; 1]d; for d 2 N: From this result we obtain large deviation principles for the number of edges per vertex, the degree distribution and the proportion of isolated vertices for the near intermediate random geometric graph models.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Wednesday, 12.12.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Botond Szabo, Universität Leiden, Niederlande:
On the fundamental understanding of distributed computation
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In recent years, the amount of available information has become so vast in certain fields of applications that it is infeasible or undesirable to carry out the computations on a single server. This has motivated the design and study of distributed statistical or learning methods. In distributed methods, the data is split amongst different administrative units and computations are carried out locally in parallel to each other. The outcome of the local computations are then aggregated into a final result on a central machine. In this talk we will consider the limitations and guarantees of distributed methods in general under communication constraints (i.e. only limited amount of bits are allowed to be transmitted between the machines) on the random design regression model. We derive minimax lower bounds, matching upper bounds and provide adaptive estimators reaching these limits. This is an ongoing joint work with Harry van Zanten.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Westfälische Wilhelms-Universität Münster
Tuesday, 11.12.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Mathias Staudigl, Universität Maastricht, Niederlande:
On the convergence of gradient-like flows with noisy gradient input
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Tuesday, 11.12.2018, 13:30 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Wolfgang Dreyer, WIAS Berlin:
Kinetic theory of incompressible non-newtonian fluids
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We consider a non-newtonian fluid consisting of a newtonian solvent and polymer molecules as solute. The kinetic theory models the polymer molecules as dumbbells and compiles dynamical equations that embody internal dumbbell interactions and interactions of the dumbbell with the surrounding fluid via its velocity gradient. The dynamical equations imply a generalized Fokker-Planck equation for a distribution function of the system of polymer molecules. Then the Fokker--Planck equation is used to derive macroscopic evolution equations for certain mean values. For given velocity gradient of the fluid, three evolution equations are needed to end up with a closed equation system for the polymer molecules. The corresponding three thermodynamic variables are the number density, the mean squared end-to end distance of a polymer molecule and the polymer stress. Finally we prove an H-theorem and solve the subtle problem of thermodynamic consistency.

Host
WIAS Berlin
Tuesday, 11.12.2018, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Jitka Machalová, Palacky University Olomouc, Tschechische Republik:
Contact problem solution for nonlinear beam and foundation by CVM
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
December 5 – 7, 2018 (WIAS-ESH)
Workshop/Konferenz: 13th International Workshop on Variational Multiscale and Stabilized Finite Elements
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Wednesday, 05.12.2018, 11:30 (WIAS-HVP-3.13)
Seminar Interacting Random Systems
Prof. Dr. Serguei Popov, University of Campinas --- UNICAMP, Brasilien:
Two-dimensional random interlacements, conditional SRW, and the cryptocurrencies
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstract
Serguei Popov will briefly describe his recent research topics mentioned in the title.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Wednesday, 05.12.2018, 10:00 (WIAS-405-406)
Forschungsseminar Mathematische Statistik
Matthias Löffler, University of Cambridge, GB:
Spectral thresholding for Markov chain transition operators
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
We consider estimation of the transition operator P of a Markov chain and its transition density p where the eigenvalues of P are assumed to decay exponentially fast. This is for instance the case for periodised multi-dimensional diffusions observed in low frequency. We investigate the performance of a spectral hard thresholded Galerkin-type estimator for P and p, discarding most of the estimated eigenpairs. Our main contribution is that we show its statistical optimality by establishing matching minimax upper and lower bounds in L^2-loss. Particularly, the effect of the dimension d on the nonparametric rate improves from 2d to d.

Further Informations
Forschungsseminar “Mathematische Statistik”

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 04.12.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Yangwen Sun, Humboldt-Universität zu Berlin:
Complete graph based online change point detection
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Friday, 30.11.2018, 13:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Steven-Marian Stengl:
Uncertainty Quantification of the Ambrosio--Tortorelli approximation in image segmentation
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this talk we want to deal with quantification of uncertainties in image segmentation based on the Mumford--Shah model. The aim is to address the error propagation of noise and other error types in the original image to the segmentation and especially the edges. We analyze therefore the in the literature well-known Ambrosio--Tortorelli approximation and discuss the existence of measurable selections of its solutions as well as sampling-based methods. We end the talk with numerical examples.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Thursday, 29.11.2018, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Prof. J.H.M. ten Thije Boonkkamp, Eindhoven University of Technology, Niederlande:
Flux vector approximation schemes for systems of conservation laws
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Conservation laws in continuum physics are often coupled, for example the continuity equations for a reacting gas mixture or a plasma are coupled through multi-species diffusion and a complicated reaction mechanism. For space discretisation of these equations we employ the finite volume method. The purpose of this talk is to present novel flux vector approximation schemes that incorporate this coupling in the discretisation. More specifically, we consider as model problems linear advection-diffusion systems with a nonlinear source and linear diffusion-reaction systems, also with a nonlinear source. The new flux approximation schemes are inspired by the complete flux scheme for scalar equations, see [1]. An extension to systems of equations is presented in [2]. The basic idea is to compute the numerical flux vector at a cell interface from a local inhomogeneous ODE-system, thus including the nonlinear source. As a consequence, the numerical flux vector is the superposition of a homogeneous flux, corresponding to the homogeneous ODE-system, and an inhomogeneous flux, taking into account the effect of the nonlinear source. The homogeneous ODE-system is either an advection-diffusion system or a diffusion-reaction system. In the first case, the homogeneous flux contains only real-valued exponentials, on the other hand, in the second case, also complex-valued components are possible, generating oscillatory solutions. The inclusion of the inhomogeneous flux makes that all schemes display second order convergence, uniformly in all parameters (Peclet and Damköhler numbers). The performance of the novel schemes is demonstrated for several test cases, moreover, we investigate several limiting cases.
References
[1] J.H.M. ten Thije Boonkkamp and M.J.H. Anthonissen, “The finite volume-complete flux scheme for advection-diffusion-reaction equations”, J. Sci. Comput., 46, 47--70, (2011).
[2] J.H.M. ten Thije Boonkkamp, J. van Dijk, L. Liu and K.S.C. Peerenboom, “Extension of the complete flux scheme to systems of comservation laws”, J. Sci. Comput., 53, 552--568, (2012).

Host
WIAS Berlin
Wednesday, 28.11.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Pierre-Étienne Druet, WIAS:
Analysis of mass transfer, Navier--Stokes equations for multicomponent fluids subject to a volume constraint
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 28.11.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Melanie Schienle, Karlsruher Institut für Technologie:
Determination of vector error correction models for different types of high-dimensionality
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We provide a shrinkage type methodology which allows for simultaneous model selection and estimation of vector error correction models (VECM) when the dimension is large and can increase with sample size. Model determination is treated as a joint selection problem of cointegrating rank and autoregressive lags under respective practically valid sparsity assumptions. We show consistency of the selection mechanism by the resulting Lasso-VECM estimator under very general assumptions on dimension, rank and error terms. Moreover, with computational complexity of a linear programming problem only, the procedure remains computationally tractable in high dimensions. For the subcase of finite dimensions, we can modify and tailor the procedure with elementwise selection consistency. For ultra-high dimensions we suggest a completely diferent pre-screening approach. We demonstrate the effectiveness of the proposed techniques in simulations and an empirical application to recent CDS data after the financial crisis.

Further Informations
Forschungsseminar “Mathematische Statistik”

Host
Universität Potsdam
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 27.11.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Yegor Klochkov, Humboldt-Universitát zu Berlin:
On uniform Hanson-Wright type inequalities for sub-Gaussian entries
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We investigate concentration of supremum of quadratic forms via entropy inequality. The problem is well understood for Gaussian vectors, and, more generally for K-concentrated vectors. We extend the result to vectors with indpenent sub-Gaussian entries with an extra logarythmic term. This is a joint work with Nikita Zhivotovskiy.

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Wednesday, 21.11.2018, 10:45 (WIAS-HVP-3.13)
Seminar Interacting Random Systems
Wen Sun, TU Berlin:
On the asymptotic distribution of nucleation times of polymerization processes
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstract
Experiments have shown that there is a sharp phase transition in polymerization: it takes long time to have small amount of stable polymers and once some amount of stable polymers appear, very quickly all particles are polymerized. Moreover, the lag time usually have a very high variance. We propose a growth-fragmentation model with a critical mass to explain these phenomenon. Particles having a mass less than this critical mass are unstable -- they are fragmented much more quicker than the larger particles. A scaling approach is used, by taking the initial total mass N as a scaling parameter and assuming that the ratio of the unstable fragmentation rates to stable fragmentation rates are of order Phi(N), which is a non-decreasing function of N. We study the time evolution of this infinite dimension process under a certain class of fragmentation distributions. We show that 1) with a proper scaling parameter, the time (T) spent for the stable polymers that generating from small particles is asymptotically exponential distributed; 2) the time for the growth of stable particles has a much smaller order than T. The exponential distribution explains the high variance and the different time scales explain the sharp phase transition. These results are proved via stochastic calculus, estimations for occupation measures on different time scales, some coupling techniques and branching processes. It is a joint work with Philippe Robert.

Further Informations
Forschungsgruppenseminar Interacting Random System

Host
WIAS Berlin
Wednesday, 21.11.2018, 10:00 (WIAS-406)
Forschungsseminar Mathematische Statistik
Prof. Markus Bibinger, Universität Marburg:
Statistical analysis of path properties of volatility
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
In this talk, we review recent contributions on statistical theory to infer path properties of volatility. The interest is in the latent volatility of an It^o semimartingale, the latter being discretely observed over a fixed time horizon. We consider tests to discriminate continuous paths from paths with volatility jumps. Both a local test for jumps at speciied times and a global test for jumps over the whole observation interval are discussed. We establish consistency and optimality properties under infill asymptotics, also for observations with additional additive noise. Recently, there is high interest in the smoothness regularity of the volatility process as confl icting models are proposed in the literature. To address this point, we consider inference on the Hurst exponent of fractional stochastic volatility processes. Even though the regularity of the volatility determines optimal spot volatility estimation methods, forecasting techniques and the volatility persistence, identifiability is an unsolved question in high-frequency statistics. We discuss a first approach which can reveal if path properties are stable over time or changing. Eventually, we discuss some recent considerations and conjectures on this open question. The related easier problem of inference on the Hurst exponent from direct discrete observations of a fractional Brownian motion is also visited.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 20.11.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Michele Coghi, WIAS (Berlin):
Pathwise McKean-Vlasov theory
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We take a pathwise approach to classical McKean-Vlasov stochastic differential equations, as found e.g. in Sznitmann. We are much inspired by Cass-Lyons (and more recent works of Bailleul et al.), but avoid all rough path complications due to our focus on additive noise. The resulting “pathwise McKean-Vlasov theory” is both simple and powerful: as applications we discuss propagation of chaos with a priori independence and exchangeability assumption; common noise and reflecting boundaries are also easy to handle in this framework, last not least we can generalise the Dawson-Gärtner large deviation result to non-Brownian noise.

Host
WIAS Berlin
November 19 – 21, 2018 (WIAS-ESH)
Workshop/Konferenz: Dynamics of Coupled Oscillator Systems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Friday, 16.11.2018, 14:00 (WIAS-ESH)
Seminar Quantitative Biomedizin
Priv.-Doz. Dr Thomas Bocklitz, Leibniz-Institut für Photonische Technologien:
Machine learning of molecular sensitive spectral and image data
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Molecular sensitive imaging modalities, like multimodal imaging, e.g. the combination of coherent-anti-Stokes Raman scattering (CARS), second-harmonic generation (SHG) and two-photon-excited fluorescence (TPEF), and Raman spectroscopic imaging feature a unique potential for disease diagnostics and treatment monitoring. In proof-of-principle studies the potential of these both imaging modalities for cancer detection and diagnostics of chronic inflammatory bowel diseases could be demonstrated. In order to utilize the huge potential of multimodal imaging and Raman spectroscopic imaging for diagnostic tasks powerful machine learning methods are necessary, which translate the experimental imaging data into medical information. This translation process is composed of image standardization, correction procedures to eliminate corrupting contributions within the acquired images, image enhancement methods and feature extraction procedures in the case of multimodal imaging. For Raman spectroscopic imaging advanced signal processing for correcting and standardizing the spectral data is necessary. After this so-called pre-processing was carried out, models like regression or classification methods can be applied to extract medical relevant information. This contribution shows, how the pre-processing have to be tailored to the measurement modalities and how the feature extraction has to be adapted to the samples and the specific diagnostic task to be solved.

Further Informations
Seminar "`Quantitative Biomedizin"'

Host
WIAS Berlin
Thursday, 15.11.2018, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Gediminas Slekas, Vilnius Gediminas Technical University, Lithuania:
Solution of Boltzmann transport equation for photo-excited electrons in semiconductor with strong electric field
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Photoconductive terahertz antennas based on a low temperature grown gallium arsenide are one of the most popular and most promising terahertz wave generators developed so far. The operation of such antennas is based on relaxation of photo-excited electrons in high electric field. Electrons excited to the conductivity band of semiconductor are moving towards antennas contacts producing electric current, which induces terahertz pulse. Whereas the electron recombination time is less than picosecond, the dynamics of electrons is highly non-linear. Drift-diffusion model is the simplest non-trivial carrier transport model which is derived as the zeroth order moment of Boltzmann transport equation. For many decades it has been used as a basic tool for simulation of semiconductor devices. However, non-linear effects, such as electron velocity overshoot, are not described by drift-diffusion equations, so higher order transport models are used to include this effect into calculations. On the one hand, higher order models still require a lot of fine-tuning and a detailed understanding of the underlying physical phenomena. On the other hand, they are still not adapted to the case when initial distribution of electrons is far from thermal equilibrium, which is true in the latter case. Monte Carlo simulation is the golden tool for solution of Boltzmann equation. However this method is very computationally expensive, so it is not very reasonable for optimization problems.

Host
WIAS Berlin
Thursday, 15.11.2018, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Jonathan Shewchuk, University of California at Berkeley, USA:
Higher-quality tetrahedral mesh generation for domains with small angles by constrained Delaunay refinement
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Most algorithms for guaranteed-quality tetrahedral mesh generation create Delaunay meshes. Delaunay triangulations have many good properties, but the requirement that all tetrahedra be Delaunay often forces mesh generators to overrefine where boundary polygons meet at small angles---that is, they produce too many tetrahedra, making them too small. Relaxing the Delaunay property makes it possible both to reduce overrefinement and to obtain higher-quality tetrahedra.
We describe a provably good algorithm that generates high-quality meshes that are *constrained* Delaunay triangulations, rather than purely Delaunay. This change has two big advantages: it allows us to generate higher-quality tetrahedra than purely Delaunay algorithms do, and it allows us to cope much more successfully with domains that have small angles. Both theory and an implementation show that our algorithm does not overrefine near small domain angles.

Host
WIAS Berlin
Wednesday, 14.11.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Hannes Meinlschmidt, RICAM, Linz, Österreich:
Maximal regularity in optimal control for quasilinear parabolic evolution equations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 14.11.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Jürgen Pilz, Universität Klagenfurt:
The interplay between random field models for Bayesian spatial prediction and the design of computer experiments
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In the first part of my talk, I will give an overview of recent work with my colleagues G. Spoeck and H. Kazianka in the area of Bayesian spatial prediction and design [1]-[5]. The Bayesian approach not only offers more flexibility in modeling but also allows us to deal with uncertain distribution parameters, and it leads to more realistic estimates for the predicted variances. Moreover, I will demonstrate how to apply copula methodology to Bayesian spatial modeling and use it to derive predictive distributions. I will also report on recent results for determining objective priors for the crucial nugget and range parameters of the widely used Matern-family of covariance functions. Briefly, I will also consider the problem of choosing an öptimal" spatial design, i.e. finding an optimal spatial configuration of the observation sites minimizing the total mean squared error of prediction over an area of interest. Our results will be illustrated by modeling environmental phenomena and designing a hyrogeological monitoring network in Upper Austria. In the second part of my talk I will report on modifying and transferring spatial random field models to make them accessible to the analysis of complex computer code. Over the last three decades, the design of computer experiments has rapidly developed as a statistical discipline at the intersection of the well established theories of DoE, stochastic processes, stochastic simulation and statistical parameter estimation, with the aim of approximating complex computer models to reproduce the behaviour of engineering, physical, biological, environmental and social science processes. We will focus on the use of Gaussian Processes (GPs) for the approximation of computer models, thereby stepping from simple parametric setups to using GPs as basis functions of additive models. Then we discuss the numerical problems associated with the estimation of the model parameters, in particular, the second-order (variance and correlation) parameters. To overcome these problems I will highlight joint recent work with my colleague N. Vollert [6] using Bayesian regularization, based on objective (reference) priors for the parameters. Finally, we will consider design problems associated with the search for numerical robustness of the estimation procedures. We illustrate our findings by modeling the magnetic field of a magnetic linear position detection system as used in the automotive industry. References: [1] H. Kazianka and J. Pilz: Bayesian spatial modeling and interpolation using copulas. Computers & Geosciences 37(3): 310-319, 2011 [2] H. Kazianka and J. Pilz: Objective Bayesian analysis of spatial data taking account of nugget andrange parameters. The Canadian Journal of Statistics 40(2): 304-327, 2012 [3] J. Pilz, H. Kazianka and G. Spoeck: Some advances in Bayesian spatial prediction and sampling design. Spatial Statistics 1: 65-81, 2012 [4] G. Spoeck and J. Pilz: Spatial sampling design based on spectral approximations of the errorprocess. In: Spatio-temporal design: Advances in Efficient Data Acquisition (W.G. Mueller and J. Mateu, Eds.), Wiley, New York 2013, 72-102 [5] G. Spoeck and J. Pilz: Simplifying objective functions and avoiding stochastic search algorithms in spatial sampling design. Front. Environ. Sci. 3:39: 1-22, 2015 [6] N. Vollert, M. Ortner and J. Pilz: Robust Additive Gaussian Process Models Using ReferencePriors and Cut-Off-Designs. To appear in: Applied Mathematical Modelling (2019), https://doi.org/10.1016/j.apm.2018.07.050

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 13.11.2018, 15:00 (WIAS-HVP-3.13)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
PhD Eric Joseph Hall, Rheinisch-Westfälische Technische Hochschule Aachen:
Causality and Bayesian network PDE for multiscale representations of porous media
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstract
Hierarchical porous media that feature properties and processes at multiple scales arise in many engineering applications including the design of novel materials for energy storage devices. Microscopic (pore-scale) properties of the media impact their macroscopic (continuum- or Darcy-scale) counterparts and understanding the relationships between processes on these two scales is essential for informing engineering decision tasks. However, microscopic properties typically exhibit complex statistical correlations that present challenges for the estimation of macroscopic quantities of interest (QoIs), e.g., global sensitivity analysis (GSA) of macroscopic QoIs with respect to microscopic material properties that respect structural constraints. We present a systematic framework for building correlations into stochastic multiscale models through Bayesian networks. This allows us to construct the joint probability density function (PDF) of model parameters through causal relationships that emulate engineering processes related to the design of hierarchical nanoporous materials. Such PDFs also serve as input for the forward propagation of parametric uncertainty; our findings indicate that the inclusion of causal relationships impacts predictions of macroscopic QoIs. To assess the impact of correlations and causal relationships between microscopic parameters on macroscopic material properties, we use a moment-independent GSA based on the differential mutual information. Our GSA accounts for the correlated inputs and complex non-Gaussian QoIs. The global sensitivity indices are used to rank the effect of uncertainty in microscopic parameters on macroscopic QoIs, to quantify the impact of causality on the multiscale model's predictions, and to provide physical interpretations of these results for hierarchical nanoporous materials.

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Tuesday, 13.11.2018, 14:15 (TUB-MA 415)
Oberseminar Nonlinear Dynamics
Dr. Hannes Stuke, Freie Universität Berlin:
A dynamical systems approach to outlier robust machine learning
more ... Location
Technische Universität, Straße des 17. Juni 136, 10623 Berlin, MA 415

Abstract
We consider a typical problem of machine learning - the reconstruction of probability distributions of observed data. We introduce the so-called gradient conjugate prior (GCP) update and study the induced dynamical system. We will explain the dynamics of the parameters and show how one can use insights from the dynamical behavior to recover the ground truth distribution in a way that is more robust agains outliers. The developed approach also carries over to neutral networks.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 13.11.2018, 13:30 (WIAS-406)
Seminar Materialmodellierung
Alex Christoph Goeßmann, Fritz-Haber-Institut der Max-Planck-Gesellschaft:
Representing crystals for kernel-based learning of their properties
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
Accurate modeling of many-body systems like crystals requires to capture their quantum-mechanical nature at the atomic scale. The solution of the associated electronic structure problem is however illusional due to the number of variables, but we obtain certain properties by computational-demanding methods like density-functional theory. In this talk, I will discuss the potential of kernel-based machine learning to circumvent this computational bottleneck and predict crystal properties. A crucial preliminary step is the representation of crystals, which has to satisfy different conditions for the learning to perform optimally.

Host
WIAS Berlin
Monday, 12.11.2018, 13:00 (WIAS-ESH)
Seminar Quantitative Biomedizin
Prof. Anna Marciniak-Czochra, Universität Heidelberg:
Post-Turing tissue pattern formation: Insights from mathematical modelling
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Cells and tissue are objects of the physical world, and therefore they obey the laws of physics and chemistry, notwithstanding the molecular complexity of biological systems. What are the mathematical principles that are at play in generating such complex entities from simple laws? Understanding the role of mechanical and mechano-chemical interactions in cell processes, tissue development, regeneration and disease has become a rapidly expanding research field in the life sciences.To reveal the patterning potential of mechano-chemical interactions, we have developed two classes of mathematical models coupling dynamics of diffusing molecular signals with a model of tissue deformation. First we derived a model based on energy minimisation that leads to 4-th order partial differential equations of evolution of infinitely thin deforming tissue (pseudo-3D model) coupled with a surface reaction-diffusion equation. The second approach (full-3D model) consists of a continuous model of large tissue deformation coupled with a discrete description of spatial distribution of cells to account for active deformation of single cells. The models account for a range of mechano-chemical feedbacks, such as signalling-dependent strain, stress, or tissue compression. Numerical simulations show ability of the proposed mechanisms to generate development of various spatio-temporal structures. We compare the resulting patterns of tissue invagination and evagination to those encountered in developmental biology. We discuss analytical and numerical challenges of the proposed models and compare them to the classical Turing patterns as well as reaction-diffusion ODE models coupling diffusion-based cell-to-cell communication with intracellular signalling.

Host
WIAS Berlin
Wednesday, 07.11.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Thomas Frenzel, WIAS Berlin:
Membrane limit for porous medium equation in weighted Wasserstein space
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We discuss the limit passage of a porous medium equation where a membrane is enclosed by two bulk-parts. The diffusion coefficient on the membrane-part scales like its thickness. By using a gradient structure with a weighted Wasserstein dissipation we perform the analysis by means of evolutionary Gamma-convergence. The major issue is then in deriving the effective dissipation potential. As a byproduct we obtain the transmission condition that couples the equations of the remaining bulk parts on the membrane with zero thickness.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 07.11.2018, 11:30 (WIAS-406)
Seminar Interacting Random Systems
András Tóbiás, Technische Universität Berlin:
Signal to interference ratio percolation for Cox point processes
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We study signal-to-interference plus noise ratio (SINR) percolation for Cox point processes, i.e., Poisson point processes with a random intensity measure. SINR percolation was first studied by O. Dousse et al. in the case of a two-dimensional Poisson point process. It is a version of continuum percolation where the connection between two points depends on the locations of all points of the point process. Continuum percolation for Cox point processes was recently studied by C. Hirsch, B. Jahnel and E. Cali. We study the SINR graph model for a stationary Cox point process in two or higher dimensions. We show that under suitable moment or boundedness conditions on the path-loss function and the intensity measure, this graph has an infinite connected component if the spatial density of points is large enough and the interferences are sufficiently reduced (without vanishing). This holds in all dimensions larger than 1 if the intensity measure is asymptotically essentially connected, and in two dimensions also if the intensity measure is only stabilizing but the connection radius is large. A prominent example of the intensity measure is the two-dimensional Poisson--Voronoi tessellation. We show that its total edge length in a given square has some exponential moments. We conclude that its SINR graph has an infinite cluster if the path-loss function is bounded and has a power-law decay of exponent at least 3.

Further Informations
Forschungsgruppenseminar Interacting Random System

Host
WIAS Berlin
Tuesday, 06.11.2018, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Prof. em. Dr. Lutz Schimansky-Geier, Humboldt-University Berlin:
Celestial mechanics of fruit flies or a theory for mushroomers
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstract
The topic of global search in complex environments have been often investigated. But a search can also be local in the sense that it is centered at a given home position. In the latter case, the searcher does not only look for a new target but is also required to regularly return to the home position. Such behavior is typical for many insects and achieves technical importance for self-navigating robotic systems. We propose a stochastic nonlinear model for local search which does not distinguish between the two aims. The dynamics bases on an unique pursuit and escape behavior of the heading from the position vector realizing thereby optimal exploration of space and the return to the home. We discuss the mechanics of the searcher and inspect the role of noise. Such randomness is present in the decision making rule of selecting the new heading direction. We consider Levy noises with different degree of discontinuity and report about steady spatial densities for the searchers. Also we report about an optimal noise intensity that a searcher finds a target at nearby places. For this noise value the required time for finding the target becomes minimal which appears to be the consequence of different relaxation processes in the spatial and the angular dynamics. Further extensions of the model are discussed during the lecture.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 06.11.2018, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Ivan Ovsyannikov, University of Hamburg:
Birth of discrete Lorenz attractors in global bifurcations
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 06.11.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Raul Tempone, Rheinisch-Westfälische Technische Hochschule Aachen:
Multilevel and Multi-index Monte Carlo methods for the McKean-Vlasov equation
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
In this talk, I will talk about our recent work where we address the approximation of functionals depending on a system of parti cles, described by stochastic differential equations (SDEs), in the mean-f ield limit when the number of particles is infinite. This problem is equiv alent to estimating the weak solution of the limiting McKean-Vlasov SDE. To that e nd, our approach uses systems with finite numbers of particles and a time-ste pping scheme. In this setting, there are two discretization parameters: the number of time steps and the number of particles. Based on these two parameters, we consider different variants of the Monte Carlo, Multilevel Monte Carlo (MLMC) and Multi-Index Monte Carlo methods and show that, based on som e assumptions that are verified numerically, we are able to achi eve a near- optimal work complexity in a typical setting.

Host
WIAS Berlin
Tuesday, 06.11.2018, 14:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. John Meddocks, École Polytechnique Fédérale de Lausanne, Switzerland:
Estimating structured precision matrices
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
In my group's work on estimating parameter sets in coarse-grain (or multi-scale) sequence-dependent models of DNA we came across the following basic problem: For a given (positive-definite) covariance matrix (in our case estimated in standard ways from Molecular dynamics simulation time series) what is the "best fit" precision, or inverse covariance, matrix under the contraint that the precision matrix must have a prescribed block banded sparsity pattern. I will describe various known, but apparently not well known, results in this and related directions, involving maximum entropy and relative entropy and likelihood principles.

Host
WIAS Berlin
Thursday, 25.10.2018, 11:30 (WIAS-406)
Seminar Interacting Random Systems
Andrea Agazzi, Duke University, USA:
Large deviations and recurrence for chemical reaction networks
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
The microscopic dynamics of well-stirred networks of chemical reactions are modeled as jump Markov processes. At large volume, one may expect in this framework to have a straightforward application of large deviation theory. This is not at all true, for the jump rates are typically neither globally Lipschitz, nor bounded away from zero, with both blowup and absorption as quite possible scenarios. In joint work with Amir Dembo and Jean-Pierre Eckmann, we utilize Lyapunov stability theory to bypass this challenges and to characterize a large class of network topologies that satisfy the full Wentzell-Freidlin theory of asymptotic rates of exit from domains of attraction. The extension of such results to the estimation of transitions times between metastable states further requires the positive recurrence of such processes. This property depends critically on dynamics at of the jump Process when the concentration of some of the components are small. Precise statements on the connection between this property and the structure of the underlying network are an active area of research.

Further Informations
Seminar Interacting Random Systems

Host
WIAS Berlin
Wednesday, 24.10.2018, 15:00 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Dan Tiba, Romanian Academy, Bucharest:
Implicit parametrizations and applications in optimization and control
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The talk will discuss necessary conditions (with less Lagrange multipliers), perturbations and general algorithms in non-convex optimization problems. Optimal control problems with mixed constraints, governed by ordinary differential equations, will also be studied in this context. The treatment is based on a recent approach to implicit systems, constructing parametrizations of the corresponding manifold, via iterated Hamiltonian equations.

Host
WIAS Berlin
Wednesday, 24.10.2018, 14:00 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Pierluigi Colli, Università di Pavia, Italien:
Shape memory alloys, phase-field models, Cahn--Hilliard systems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The title collects some special keywords for research fields in the framework of phase transitions and free boundary problems describing an important research area receiving attention, interest, and a number of significant contributions from Jürgen Sprekels. This talk will offer the speaker the possibility of reviewing some models, problems, and results in the analysis and optimal control of the related systems of partial differential equations. And, more than that, it will be a pleasure for him to recall some thirty years enjoying math by his collaboration with J. Sprekels.

Host
WIAS Berlin
Wednesday, 24.10.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Evgeny Stepanov, Russian Academy of Sciences, Russische Föderation:
Hybrid control in a model from molecular biology: Between continuous and discrete
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The central argument of the talk will be the rather modern concept in automated control: Hybrid controls through finite state machines in continuous dynamical models. It will be shown that hybrid systems present a highly complicated dynamics. Then a class of largely open problems arizing from molecular biology will be presented and studied, where the hybrid system comes out from a extra- ordinarily simple and completely classical models based on ODEs without any discrete object incorporated.

Further Informations
Forschungsseminar “Mathematische Statistik”

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 23.10.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexey Naumov, National Research University Higher School of Economics, Russische Föderation:
Gaussian approximations for maxima of large number of quadratic forms of high-dimensional random vectors
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Tuesday, 23.10.2018, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Dan Tiba, Romanian Academy, Bucharest:
Optimal control methods in shape optimization
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Wednesday, 17.10.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Artur Stephan, WIAS Berlin:
On reaction systems with different time scales
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 17.10.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Mark Podolskij, Aarhus University, Denmark:
Statistical inference for fractional models
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In recent years, fractional and moving average type models have gained popularity in economics and finance. Most popular examples include fractional Brownian/stable motion, rough volatility models and Hawkes processes. In this talk we will review some existing estimation methods and present new theoretical results.

Host
WIAS Berlin
Universität Potsdam
Humboldt-Universität zu Berlin
Tuesday, 16.10.2018, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Jan Sieber, University of Exeter, UK:
A nonsmooth saddle-node in a non-autonomous dynamical system
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 16.10.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Alexander Gasnikov, WIAS Berlin:
Unified view on accelerated methods for structural convex optimization problems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Tuesday, 16.10.2018, 13:30 (WIAS-406)
Seminar Materialmodellierung
Dr. Arik Yochelis, Ben-Gurion University of the Negev, Israel:
From solvent free to dilute electrolytes: A unified continuum approach
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Tuesday, 16.10.2018, 10:15 (WIAS-406)
Seminar Materialmodellierung
Dr. Charlotte Kuhn/Dr. Alexander Schlüter, Technische Universität Kaiserslautern:
Phase field modelling of fracture -- From a mechanics point of view
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Further Informations
Seminar Materialmodellierung

Host
WIAS Berlin
October 8 – 10, 2018 (WIAS-ESH)
Workshop/Konferenz: AMaSiS 2018: Applied Mathematics and Simulation for Semiconductors
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
University of Massachusetts
Tuesday, 25.09.2018, 15:00 (WIAS-HVP-3.13)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Franz Besold, Humboldt-Universität zu Berlin:
Adaptive clustering using kernel density estimators
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstract
We investigate statistical properties of a clustering algorithm suggested by Ingo Steinwart, Bharath K. Sriperumbudur and Philipp Thomann that receives level set estimates from a kernel density estimator and then estimates the first split in the density level cluster tree if such a split is present or detects the absence of such a split. Key aspects of the analysis include finite sample guarantees, consistency, rates of convergence, and an adaptive data-driven strategy for choosing the kernel bandwidth. For the rates and the adaptivity we do not need continuity assumptions on the density such as Hölder continuity, but only require intuitive geometric assumptions of non-parametric nature.

Host
WIAS Berlin
September 24 – 28, 2018 (WIAS-ESH)
Workshop/Konferenz: Analysis of Evolutionary and Complex Systems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Universität Stuttgart
University of Warwick
Tuesday, 18.09.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. M. Thamban Nair, Indian Institute of Technology Madras, Indien:
An inverse problem in parabolic PDEs
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Tuesday, 18.09.2018, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Alexander Kruger, Federation University Ballarat, Australia:
Extremal principle revisited
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Thursday, 13.09.2018, 15:00 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Mine Akbas, Duzce University, Türkei:
The analogue of grad-div stabilization in DG methods for incompressible flows: limiting behavior and extension to tensor-product meshes
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Grad-div stabilization is a classical remedy in conforming mixed finite element methods for incompressible flow problems, for mitigating velocity errors that are sometimes called poor mass conservation. Such errors arise due to the relaxation of the divergence constraint in classical mixed methods and are excited whenever the spacial discretization has to deal with comparably large and complicated pressures. In this contribution, an analogue of grad-div stabilization for Discontinuous Galerkin methods is studied. Here, the key is the penalization of the jumps of the normal velocities over facets of the triangulation, which controls the measure-valued part of the distributional divergence of the discrete velocity solution. Our contribution is twofold: first, we characterize the limit for arbitrarily large penalization parameters, which shows that the stabilized nonconforming Discontinuous Galerkin methods remain robust and accurate in this limit; second, we extend these ideas to the case of non-simplicial meshes; here, broken grad-div stabilization must be used in addition to the normal velocity jump penalization, in order to get the desired pressure robustness effect. Several numerical examples illustrate the theory and show their relevance for the simulation of practical, nontrivial flows.

Host
WIAS Berlin
Thursday, 13.09.2018, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Vojtěch Miloš, Charles University Prague:
Thermodynamics and electrochemical impedance spectroscopy - 1D model of YSZ electrolyte and triple phase boundary interface
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Electrochemical impedance spectroscopy (EIS) together with reliable models provides a powerful tool for analysing processes taking place in solid oxide cells (SOC). The presented model is derived within the framework of non-equilibrium thermodynamics [1, 2]. It is a suitable tool for description of the fundamental processes in charged mixtures [3, 4, 5], especially on interfaces, such as surface adsorption, bulk and surface diffusion, electrochemical reactions and formation of charged double layers. Special attention is dedicated to the thermodynamically consistent derivation of the boundary conditions and to handling of the electrical double layer. The contribution contains a derivation of 1D model for YSZ electrolyte and tripple boundary interface and also includes numerical solution of model partial differential equations. The implementation of finite element method (FEM) is done using open source library FEniCS. In order to explore the parametric space, EIS was simulated for a wide range of kinetic parameters of the model.
References:
1. De Groot, S. R., Mazur, P. Non-equilibrium thermodynamics., 1984.
2. Guhlke, Clemens. “Theorie der elektrochemischen Grenzfläche”, TU Berlin (2015).
3. Dreyer, W., Guhlke, C., Landstorfer, M. Electrochem. Commun. 43 (2014): 75-78.
4. Dreyer, W., Clemens, Guhlke, C., Müller, R. Ph. Chem. Chem. Ph. 17.40 (2015): 27176-27194.
5. Landstorfer, M., Guhlke, C., Dreyer, W. Electrochimica Acta 201 (2016): 187-219.


Host
WIAS Berlin
Tuesday, 11.09.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Franz Besold, Humboldt-Universität zu Berlin:
Adaptive clustering using kernel density estimators
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We investigate statistical properties of a clustering algorithm suggested by Ingo Steinwart, Bharath K. Sriperumbudur and Philipp Thomann that receives level set estimates from a kernel density estimator and then estimates the first split in the density level cluster tree if such a split is present or detects the absence of such a split. Key aspects of the analysis include finite sample guarantees, consistency, rates of convergence, and an adaptive data-driven strategy for choosing the kernel bandwidth. For the rates and the adaptivity we do not need continuity assumptions on the density such as Hölder continuity, but only require intuitive geometric assumptions of non-parametric nature.

Host
WIAS Berlin
Monday, 03.09.2018, 14:00 (WIAS-ESH)
INSTITUTSKOLLOQUIUM
Prof. Dr. Bernd Witzigmann, University of Kassel, Germany:
Carrier Injection Efficiency and Impurity Modeling in III-nitride LEDs (joint work with H. Fülle and F. Römer)
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
III-nitride based multiple quantum well structures are the active region of choice for LEDs and lasers from the ultraviolet to the visible spectral regime. For maximum efficiency, the electron and hole carrier densities need to be distributed uniformly in the quantum wells at all relevant operating conditions. We analyze the carrier injection and distribution in polar and semi-polar LEDs using a microscopic carrier transport model. It is based on drift-diffusion currents, and quantum wells are treated as scattering centers, with self-consistent kp-band structure calculation for the quantum well carriers. Tunneling currents at hetero interfaces are included via an effective potential model. Impurities such as Si, Mg and O are activated according to a Schottky-type model. Their distribution is found from experimental SIMS data. The activation mode includes both the impurity density as well as the local electric field (Poole-Frenkel effect). Comparison to experimental data reveals the following findings: the hole injection efficiency depends critically on the proximity of the Mg dopants to the p-side quantum well. At the same time, electron leakage is suppressed by ionized Mg atoms forming an electrostatic barrier, which is reduced with rising current due to saturation of the acceptor levels. Impurities that form shallow donors (e.g. oxygen) also hamper the hole injection efficiency as they reduce the Mg ionization rate. In UV LEDs, special care has to be taken for electron injection into the deep quantum wells. Therefore a high performance LED requires nanoscale control of intentional and unintentional impurity distribution. Finally, it is shown that the ideality factor of the pin-diode contains information on the injection homogeneity of the LED.

Host
WIAS Berlin
Thursday, 23.08.2018, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Markus Kantner, WIAS Berlin:
Hybrid quantum-classical modeling of quantum dot based single-photon sources
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Thursday, 26.07.2018, 15:00 (WIAS-406)
FG Stochastische Systeme mit Wechselwirkung
Prof. Alexander Drewitz, Universität zu Köln:
Phase transitions in some percolation models with long-range correlations on general graphs
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We consider two fundamental percolation models with long-range correlations on reasonably general and well-behaved transient graphs: The Gaussian free field and (the vacant set) of Random Interlacements. Both models have been the subject of intensive research during the last years and decades, on $Z^d$ as well as on some more general graphs. We consider their percolation phase transition and investigate a couple of interesting properties of their critical parameters, in particular the existence of a phase transition. This talk is based on joint works with A. Prevost (Koeln) and P.-F. Rodriguez (Los Angeles).

Further Informations
FG 5 Seminar

Host
WIAS Berlin
Wednesday, 18.07.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Jan van Waaij, Humboldt Universität zu Berlin:
Adaptive nonparametric Bayesian methods for SDEs
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We consider continuous time observations (Xt_0le tle T of the SDE dXt = Theta(Xt)dt + dWt: Our goal is estimating the unknown drift function Theta. Due to their numerical advantages, Bayesian methods are often used for this. In this talk I discuss optimal rates of convergence for these methods. I will start with an introduction to Bayesian methods for SDEs and what sufficient con- ditions are for posterior convergence. I show that the sufficient conditions are satisfied for the Gaussian process prior, which leads to optimal convergence rates, provided the smoothness of Theta matches the smootness of the Gaussian process. Adaptivity can be obtained by equipping the hyperparameter(s) of the Gaussian process prior with an additional prior. We discuss several choices for such hyperparameters. A new promising approach to obtain adaptivity is empirical Bayes. Here the optimal hyperparameters for the Gaussian process prior are first esti- mated from the data and then the prior with those hyperparameters plugged- in is used for the inference. This talk is based on van Waaij, 2018 and joint work with Frank van der Meulen (TU Delft), Moritz Schauer (Leiden University) and Harry van Zanten (University of Amsterdam).

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Thursday, 12.07.2018, 10:00 (WIAS-ESH)
Seminar Laserdynamik
Prof. Dmitry Turaev, Imperial College London, GB:
On positive metric entropy conjecture
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Wednesday, 11.07.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Martin Heida, WIAS Berlin:
Dynamic and hysteretic boundary conditions for the Mullins-Sekerka equation
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 11.07.2018, 11:00 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Ernesto Meneses Rioseco, Leibniz-Institut für Angewandte Geophysik, Hannover:
Modeling, simulation and optimization of geothermal reservoir -- THMC (thermo-hydraulical, mechanical, chemical) processes involved
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Geothermal energy constitutes an important renewable energy that can significantly contribute to the energy transition in Germany. The optimization of the geothermal energy extraction in the underground is a multi-variable and multi-scale optimization problem that has not been extensively addressed in the scientific community. Understanding and minimizing the risks, in particular induced seismicity, that the exploitation of deep geothermal energy may pose is of major public and scientific concern.
Robust mathematical models and numerical methods are required to capture the complex interplay of thermal, hydraulic, mechanical and chemical (THMC) processes in heterogeneous and anisotropic medium domains. In addition, the distribution and implementation of material properties in topologically complex 3D modeling domains is an ongoing effort. Considerable effort is required to realistically implement the complex architecture that results from highly heterogeneous and anisotropic geologic media.
This work presents a general overview of the mathematical formulations of the transport phenomena of mass, energy and momentum in porous and fractured geothermal reservoirs and emphasizes the current challenges faced for the simulation of the main risks (e.g. induced seismicity) that result from the complex interaction of THMC processes during the operation of geothermal plants and the need of complex optimization methods.
This presentation aims at encouraging the collaboration between the Leibniz Institute of Applied Geophysics (LIAG) and the Weierstrass Institute for Applied Analysis and Stochastics (WIAS) within the framework of the Leibniz MMS network.

Host
WIAS Berlin
Monday, 09.07.2018, 14:00 (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Nils Berglund, Iniversite d'Orleans, Universite de Tours, CNRS, Frankreich:
Metastable dynamics of Allen--Cahn SPDEs on the torus
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Stochastic processes subject to weak noise often show a metastable behaviour, meaning that they converge to equilibrium extremely slowly; typically, the convergence time is exponentially large in the inverse of the variance of the noise (Arrhenius law). In the case of finite-dimensional Ito stochastic differential equations, the large-deviation theory developed in the 1970s by Freidlin and Wentzell allows to prove such Arrhenius laws and compute their exponent. Sharper asymptotics for relaxation times, including the prefactor of the exponential term (Eyring--Kramers laws) are known, for instance, if the stochastic differential equation involves a gradient drift term and homogeneous noise. One approach that has been very successful in proving Eyring--Kramers laws, developed by Bovier, Eckhoff, Gayrard and Klein around 2005, relies on potential theory. I will describe Eyring--Kramers laws for some parabolic stochastic PDEs such as the Allen--Cahn equation on the torus. In dimension 1, an Arrhenius law was obtained in the 1980s by Faris and Jona-Lasinio, using a large-deviation principle. The potential-theoretic approach allows us to compute the prefactor, which turns out to involve a Fredholm determinant. In dimensions 2 and 3, the equation needs to be renormalized, which turns the Fredholm determinant into a Carleman--Fredholm determinant. Based on joint work with Barbara Gentz (Bielefeld), and with Giacomo Di Gesù (Vienna) and Hendrik Weber (Warwick).

Host
Freie Universität Berlin
WIAS Berlin
Thursday, 05.07.2018, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Nina Owschimikow, TU Berlin:
Carrier dynamics in InAs submonolayer quantum dots
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Wednesday, 04.07.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Jan Mankau, Technische Universität Dresden:
A descent step algorithm based on generalized gradients on sets
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
here

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Tuesday, 03.07.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Paolo Pigato, WIAS Berlin:
Asymptotic analysis of rough volatility models
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Friday, 29.06.2018, 14:30 (WIAS-ESH)
Halbleiterseminar
Dr. Miguel Caro, Aalto University, Finnland:
Growth of amorphous carbon simulated with a machine-learning based interatomic potential
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
WIAS-Halbleiterseminar

Host
WIAS Berlin
Friday, 29.06.2018, 14:00 (WIAS-ESH)
Halbleiterseminar
Prof. Dr. Morten Willatzen, Technical University of Denmark, Dänemark:
Strain and symmetry effects in Bi2Se3 thin films using the k·p method
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
WIAS-Halbleiterseminar

Host
WIAS Berlin
Wednesday, 27.06.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Leonid Berlyand, Pennsylvania State University, USA:
Free boundary PDE models of active gels
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
here

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 27.06.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Stanislav Nagy, Charles University, Prague:
Statistical depth for functional data
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The concept of data depth is an attempt to generalize quantiles to complex data (random vectors, random functions, or distributions on manifolds and graphs). The main idea is, for a general space S, to assign to any points from S its depth D(s; P) with respect to a probability distribution P on S. The depth D(s; P) says how "centrally located" s is with respect to P. The point maximizing D(:; P) over the sample space is a generalized median; loci of points with high depth constitute inner depth-quantile regions of P. We study depth designed for functional data, and its general framework. We show that most known functional depths can be classi ed into few groups, within which they share similar theoretical properties. We focus on uniform sample consistency results, and demonstrate that some well-known approaches to depth assessment are hardly theoretically adequate.

Host
Universität Potsdam
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 26.06.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexandra Suvorikova, WIAS Berlin:
CLT in 2-Wasserstein space
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Tuesday, 26.06.2018, 13:30 (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Traian Iliescu, Virginia Tech, USA:
Physically-constrained data-driven correction for reduced order modeling of fluid flows
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We propose a data-driven correction reduced order model (DDC-ROM) framework for the numerical simulation of fluid flows, which can be formally written as DDC-ROM = Galerkin-ROM + Correction. The new DDC-ROM is constructed by using ROM spatial filtering and data-driven ROM closure modeling (for the Correction term). Furthermore, we propose a physically-constrained DDC-ROM (CDDC-ROM), which aims at improving the physical accuracy of the DDC-ROM. The new physical constraints require that the CDDC-ROM operators satisfy the same type of physical laws (i.e., the Correction term's linear component should be dissipative and the Correction term's nonlinear component should conserve energy) as those satisfied by the fluid flow equations. To implement these physical constraints, in the data-driven modeling step, we replace the unconstrained least squares problem with a constrained least squares problem. We perform a numerical investigation of the new CDDC-ROM and standard DDC-ROM for a 2D channel flow past a circular cylinder at Reynolds numbers Re=100, Re=500, and Re=1000. To this end, we consider a reproductive regime as well as a predictive (i.e., cross-validation) regime in which we use as little as 50 % of the original training data. The numerical investigation clearly shows that the new CDDC-ROM is significantly more accurate than the DDC-ROM in both regimes.

Host
WIAS Berlin
June 25 – 27, 2018 (TU-H2036)
Workshop/Konferenz: Interplay of Random Media and Stochastic Interface Models (RMSI2018)
more ... Location
Technische Universität, Straße des 17. Juni 135, 10623 Berlin, Main building, Room H 2036

Host
Technische Universität Berlin
Wednesday, 20.06.2018, 14:30 (WIAS-ESH)
INSTITUTSKOLLOQUIUM
Prof. Dr. Michael C. Ferris, Computer Sciences Department and Wisconsin Institute for Discovery, University of Wisconsin-Madison, USA:
Modelling 100 percent renewable electricity
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In the past few decades, power grids across the world have become dependent on markets that aim to efficiently match supply with demand at all times via a variety of pricing and auction mechanisms. These markets are based on models that capture interactions between producers, transmission and consumers. Energy producers typically maximize profits by optimally allocating and scheduling resources over time. A dynamic equilibrium aims to determine prices and dispatches that can be transmitted over the electricity grid to satisfy evolving consumer requirements for energy at different locations and times. Computation allows large scale practical implementations of socially optimal models to be solved as part of the market operation, and regulations can be imposed that aim to ensure competitive behaviour of market participants. The recent explosion in the use of renewable supply such as wind, solar and hydro has led to increased volatility in this system. We develop models that aim to ensure enough generation capacity for the long term under various constraints related to environmental concerns, and consider the recovery of costs for this enhanced infrastructure. We demonstrate how risk can impose significant costs on the system that are not modeled in the context of socially optimal power system markets and highlight the use of contracts to reduce or recover these costs. We also outline how battery storage can be used as an effective hedging instrument.

Host
WIAS Berlin
Wednesday, 20.06.2018, 10:00 (WIAS-406)
Forschungsseminar Mathematische Statistik
Prof. Zuoqiang Shi, Tsinghua University, Beijing, China:
PDE-based methods for Interpolation on High Dimensional Point Cloud
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
Interpolation on high dimensional point cloud provides a fundamental model in many data analysis and machine learning problems. In this talk, we will present some PDE based methods to do interpolation on point cloud. Applications in image processing and data analysis are shown to demonstrate the performance of our methods.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 19.06.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Zuoqiang Shi, Tsinghua University, Beijing, China:
Low dimensional manifold model for image processing
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
In this talk, I will introduce a novel low dimensional manifold model for image processing problem. This model is based on the observation that for many natural images, the patch manifold usually has low dimension structure. Then, we use the dimension of the patch manifold as a regularization to recover the original image. Using some formula in differential geometry, this problem is reduced to solve Laplace-Beltrami equation on manifold. The Laplace-Beltrami equation is solved by the point integral method. Numerical tests show that this method gives very good results in image inpainting, denoising and super-resolution problem. This is joint work with Stanley Osher and Wei Zhu.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 19.06.2018, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Guanghui Hu, Beijing Computational Science Research Center, China:
Outgoing radiation conditions for the Helmholtz equation in inhomogeneous periodic media
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Monday, 18.06.2018, 15:00 (WIAS-406)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Johannes Pfefferer, TU München:
hp-finite elements for fractional diffusion
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
In this talk we introduce and analyze a numerical scheme based on hp-finite elements to solve boundary value problems involving the spectral fractional Laplacian. The approach is based on a reformulation of the problem posed on a semi-infinite cylinder in one more spatial dimension. After a suitable truncation of this cylinder, the resulting problem is discretized with linear finite elements in the original domain and with hp-finite elements in the extended direction. The proposed approach yields a reduction of the computational complexity in terms of degrees of freedom and even has slightly improved convergence properties compared to the state-of-the-art discretization using linear finite elements for both the original domain and the extended direction. The performance of the method is illustrated by numerical experiments.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
Humboldt-Universität zu Berlin
WIAS Berlin
June 18 – 20, 2018 (WIAS-ESH)
Workshop/Konferenz: Nonlinear Dynamics in Semiconductor Lasers
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Thursday, 14.06.2018, 15:00 (WIAS-405-406)
Seminar Numerische Mathematik
Prof. Liyong Shen, University of the Chinese Academy of Sciences, Beijing:
Implicitizing rational tensor product surfaces using the resultant of three moving planes
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
Implicitizing rational surfaces is a fundamental computational task in Computer Graphics and Computer Aided Design. Ray tracing, collision detection, and solid modeling all benefit from implicitization procedures for rational surfaces. Recently a popular method is based on analysis of moving planes (syzygies): construct the implicit matrix using moving planes or compute the resultant of μ-basis. The moving planes method relies on the construction of appropriate moving planes. The μ-bases for rational surfaces are difficult to compute. Moreover, μ-bases for a rational surface often have high degrees, so these resultants generally contain many extraneous factors.
Here we develop fast algorithms to implicitize rational tensor product surfaces by computing the resultant of three moving planes corresponding to three syzygies with low degrees. These syzygies are easy to compute, and the resultants of the corresponding moving planes generally contain fewer extraneous factors than the resultants of the moving planes corresponding to μ-bases. We predict and compute all the possible extraneous factors that may appear in these resultants.

Host
WIAS Berlin
Thursday, 14.06.2018, 14:00 (WIAS-405-406)
Seminar Numerische Mathematik
Dr. Myfanwy Evans, TU Berlin:
Surfaces and tangling
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
In biological systems, hyperbolic surfaces are readily identifiable as critical to function. This talk will introduce a few interesting systems that motivate thinking in this direction, followed by a more formal mathematical process that aims to probe the realm of complicated three-dimensional structure, in particular highly tangled materials.

Host
WIAS Berlin
June 14 – 16, 2018 (WIAS-ESH)
Workshop/Konferenz: 9th Annual ERC Berlin-Oxford Young Researchers Meeting on Applied Stochastic Analysis
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Technische Universität Berlin
University of Oxford
Wednesday, 13.06.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Markus Mittnenzweig, WIAS Berlin:
Hydrodynamic limit and large deviations for reaction--diffusion master equations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 13.06.2018, 13:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Kathrin Welker, Universität Trier:
Constrained shape optimization problems in shape spaces
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Shape optimization problems arise frequently in technological processes which are modelled in the form of partial differential equations (PDEs) or variational inequalities (VIs). In many practical circumstances, the shape under investigation is parametrized by finitely many parameters, which on the one hand allows the application of standard optimization approaches, but on the other hand limits the space of reachable shapes unnecessarily. In this talk, the theory of shape optimization is connected to the differential-geometric structure of shape spaces. In particular, efficient algorithms in terms of shape spaces and the resulting framework from infinite dimensional Riemannian geometry are presented. In this context, the space of H^1/2-shapes is defined. The H^1/2-shapes are a generalization of smooth shapes and arise naturally in shape optimization problems. Moreover, VI constrained shape optimization problems are treated from an analytical and numerical point of view in order to formulate approaches aiming at semi-smooth Newton methods on shape vector bundles. Shape optimization problems constrained by VIs are very challenging because of the necessity to operate in inherently non-linear and non-convex shape spaces. In classical VIs, there is no explicit dependence on the domain, which adds an unavoidable source of non-linearity and non-convexity due to the non-linear and non-convex nature of shape spaces.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 13.06.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Alain Celisse, Universite des Sciences et Technologies de Lille, France:
Early stopping rule and discrepancy principle in reproducing kernel Hilbert spaces
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The main focus of this work is on the nonparametric estimation of a regression function by means of reproducing kernels and several iterative learning algorithms such as gradient descent, spectral cut-off, Tikhonov regularization,... First, we exploit the general framework of filter estimators to provide a unified analysis of these different algorithms. With the Tikhonov regularization, we will discuss the in uence of the parametrization on the interaction between the condition number of the Gram matrix and the number of iterations. More generally, we also discuss existing links between the qualification assumption and the used filter estimators. Second, we introduce an early stopping rule derived from the so-called discrepancy principle. Its behavior is compared with that one of other existing stopping rules and analyzed through the understanding of the dependence of the empirical risk with respect to in uential parameters (Gram matrix eigenvalues, cumulative step size, initialization). An oracle-type inequality is derived to quantify the finite-sample performance of the proposed stopping rule. The practical performance of the procedure is also empirically assessed from several simulation experiments.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 12.06.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Pavel Dvurechensky, WIAS Berlin:
Distributed calculation of Wasserstein barycenters
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We study the semi-discrete optimal transport problem of decentralized distributed computation of a discrete approximation for regularized Wasserstein barycenter of a finite set of continuous probability measures distributedly stored over a network. Particularly, we assume that there is a network of agents/machines/computers where each agent holds a private continuous probability measure, and seek to compute the barycenter of all the measures in the network by getting samples from its local measure and exchanging information with its neighbors. Motivated by this problem, we develop a novel accelerated primal-dual stochastic gradient descent method for general stochastic convex optimization problems with linear equality constraints, and modify it for decentralized distributed optimization setting to generate a new algorithm for the distributed semi-discrete regularized Wasserstein barycenter problem. The proposed algorithm can be executed over arbitrary networks that are undirected, connected and static, using the local information only. Moreover, we show explicit non-asymptotic convergence rates in terms of the problem parameters. Finally, we show the effectiveness of our method on the distributed computation of the regularized Wasserstein barycenter of univariate Gaussian and von Mises distributions; as well as some applications to image aggregation.

Host
WIAS Berlin
Tuesday, 12.06.2018, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Michael Peters, Eidgenössische Technische Hochschule Zürich, Schweiz:
Uncertainty quantification for elliptic PDEs
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Monday, 11.06.2018, 16:00 (WIAS-406)
Seminar Laserdynamik
Iva Bačić, University of Belgrade:
Stochastic dynamics in systems of adaptively coupled active rotators
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Monday, 11.06.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Dorothee Knees, Universität Kassel:
Rate-independent processes with time-discontinuous data
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Rate-independent processes typically exhibit nonsmooth behavior even if the given data are smooth in time. If the underlying energy functional is nonconvex, then discontinuous solutions appear. Meanwhile, there is a wide variety of solution concepts dealing with such discontinuities. However, in these investigations it is typically assumed that the given data (applied loads in the mechanical context) are in some sense differentiable with respect to time. But this excludes processes, where for instance the loads are switched on or off instantaneously. The aim of this lecture is to discuss for a prototypical rate-independent model with a nonconvex energy functional a solution concept that allows for discontinuous data. The idea is to start from a vanishing viscosity procedure and to identify as precisely as possible the limit process. If time permits the results are put into relation with the results from [1], where the convex case is treated in the framework of Kurzweil-integrals.
The lecture relies on joint work with Chiara Zanini (Politecnico di Torino) and discussions with Pavel Krejci.
[1] P. Krejci, M. Liero, Rate independent Kurzweil processes, Appl. Math., Praha, 54(2):117--145,2009.

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 06.06.2018, 16:00 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Seungchan Ko, Oxford University, GB:
Analysis and approximation of incompressible chemically reacting non-Newtonian fluids
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
here

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 06.06.2018, 10:00 (WIAS-HVP-3.13)
Forschungsseminar Mathematische Statistik
Igor Cialenco, Illinois Institute of Technology:
Parameter estimation problems for parabolic SPDEs
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstract
In the rst part of the talk we will discuss the parameter estimation problem using Bayesian approach for the drift coecient of some linear (parabolic) SPDEs driven by a multiplicative noise of special structure. We assume that one path of the rst N Fourier modes of the solution are continuously observed over a nite time interval, and we derive Bayesian type estimators for the drift coefficient. As custom for Bayesian statistics, we prove a Bernstein--Von Mises theorem for the posterior density, and consequently, we derive some asymptotic properties of the proposed estimators, as N goes to in nity. In the second part of the talk we will study parameter estimation problems for discretely sampled SPDEs. We will discuss some general results on derivation of consistent and asymptotically normal estimators based on computation of the p-variations of stochastic processes and their smooth perturbations, that consequently are conveniently applied to SPDEs. Both the drift and the volatility cofficients are estimated using two sampling schemes - observing the solution at a xed time and on a discrete spatial grid, and at a xed space point and at discrete time instances of a nite interval. The theoretical results will be illustrated via numerical examples.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 05.06.2018, 15:15 (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Prof. Dr. Jens Starke, Universität Rostock:
Multiscale analysis of collective behaviour in particle systems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 05.06.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Priv.-Doz. Dr. Peter Mathé, WIAS Berlin:
Nyström subsampling for regression functions of low smoothness
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Tuesday, 05.06.2018, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Daniel Walter, Technische Universität München:
Sparse sensor placement for inverse problems: From finite to infinite dimensional parameters
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Wednesday, 30.05.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. José Francisco Rodrigues, University of Lisbon, Portugal:
Evolutionary quasi-variational and variational inequalities with constraints on the derivatives
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this talk I shall present a general framework for the study of the existence of quasi-variational and variational solutions to a class of nonlinear evolution systems in convex sets of Banach spaces describing constraints on a linear combination of partial derivatives of the solutions, which is a joint work with Lisa Santos and Fernando Miranda. The quasi-linear operators are of monotone type, but are not required to be coercive for the existence of weak solutions, which is obtained by a double penalisation/regularisation for the approximation of the solutions. In the case of time-dependent convex sets that are independent of the solution, we show also the uniqueness and the continuous dependence of the strong solutions of the variational inequalities, extending previous results to a more general framework and including several models that will be surveyed.

Further Informations
Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 30.05.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Florian Schäfer, California Institute of Technology, USA:
Compression, inversion, and approximate PCA of dense kernel matrices at near-linear computational complexity
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Many popular methods in machine learning, statistics, and uncertainty quantification rely on priors given by smooth Gaussian processes, like those obtained from the Matern covariance functions. Furthermore, many physical systems are described in terms of elliptic partial differential equations. Therefore, implicitely or explicitely, numerical simulation of these systems requires an ecient numerical representation of the corresponding Green's operator. The resulting kernel matrices are typically dense, leading to (often prohibitive) O (N^2) or O(N^3) computational complexity. In this work, we prove rigorously that the dense N x N kernel matrices obtained from elliptic boundary value problems and measurement points distributed approximately uniformly in a d-dimensional domain can be Cholesky factorised to accuracy  in computational complexity O(N log^2(N) log^2d(N/e)) in time and O (N log(N) log d(N/e)) in space. For the closely related Matern covariances we observe very good results in practise, even for parameters corresponding to non-integer order equations. As a byproduct, we obtain a sparse PCA with near-optimal low-rank approximation property and a fast solver for elliptic PDE. We emphasise that our algorithm requires no analytic expression for the covariance function. Our work is inspired by the probabilistic interpretation of the Cholesky factorisation, the screening effect in spatial statistics, and recent results in numerical homogenisation.

Further Informations
Forschungsseminar “Mathematische Statistik”

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 29.05.2018, 15:15 (FU Berlin)
Oberseminar Nonlinear Dynamics
Murad Banaji, Middlesex University London, England:
Inheritance of behaviours in bio/chemical networks
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstract
Consider the following imprecise questions about dynamical systems with some network structure: If a small network is embedded in a larger one, then must the larger one "inherit" some behaviours of the smaller one? Can the presence of certain "motifs" be sensibly used to make predictions about dynamical behaviours of a network? Once such questions are phrased precisely, the answers turn out to depend on the class of networks studied, the dynamical behaviour of interest and, crucially, the chosen notion of ßubnetwork". My talk will focus on some recent results on chemical reaction networks (CRNs), where the behaviours in question are multistationarity and oscillation. Network modifications such as adding or deleting reactions, adding or deleting species from reactions, and inserting intermediates into a reaction are considered for their effects on multistationarity and oscillation. For example, it can be shown that both behaviours are inherited in the induced subnetwork partial ordering on fully open CRNs, but the same does not hold for general CRNs. Under certain mild assumptions, growing a CRN by inserting intermediates into reactions preserves the capacity for multistationarity in a CRN. The theorems are proved using essentially local techniques -- the implicit function theorem and regular and singular perturbation theory -- and hold for various kinetics including (but not exclusively) mass action kinetics. The results on multisationarity are joint with Casian Pantea.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 29.05.2018, 13:30 (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Grigory Panasenko, University of Lyon, Frankreich & University of Chile:
Asymptotic analysis of wave propagation in a laminated beam with contrasting stiffness of the layers
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The wave equation in a thin laminated beam is considered in the case when the ratio of the thickness and the length is a small parameter e, while the ratio of the stiffness coefficients of the layers is a great parameter w. The question of an approximation of such a beam by the one dimensional models is discussed. It is shown that the classical homogenized approximation is valid for the case of sufficiently small value of the product of e square by w. If this product is not small then the classical model should be replaced by a multicomponent homogenization model analogous to [1, 2]. The waves in such multicomponent beam model can have multiple wave velocities. This result generalizes [3] Section 2.6.

[1] Panasenko, G.P. “Averaging of processes in strongly in homogeneous media”, Doklady Akademii Nauk SSSR, 1988, 298, 1, 76--79 (in Russian). English transl. in Soviet Phys. Dokl., 1988, 33.
[2] Panasenko, G.P. “Multicomponent homogenization of processes in strongly non-homogeneous structures” Mathematics USSRSbornik, 1990, 181, 1, 134--142 (in Russian); English transl. in Math. USSR Sbornik, 1991, 69, 1, 143--153.
[3] Panasenko, G.P. “Multi-Scale Modelling for Structures and Composites”, Springer, Dordrecht, 2005, 398 pp.

Host
WIAS Berlin
Monday, 28.05.2018, 14:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Francisco José Romero Hinrichsen, ETH Zurich, Schweiz:
Dynamical super-resolution with applications to ultrafast ultrasound
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Recently there has been a successful development in ultrasound imaging, increasing significantly the sampling rate and therefore enhancing this imaging's capacities. In particular, for vessel imaging, the use of microbubble tracking allows us to super-resolve blood vessels, and by estimating the particles' speeds inside them, it is possible to calculate the vessels' diameters. In this context, we model the microbubble tracking problem, formulating it in terms of a sparse spike recovery problem in the phase space (the position and velocity space), that allows us to obtain simultaneously the speed of the microbubbles and their location. This leads to an L1 minimization algorithm for point source tracking, that promises to be faster than current alternatives.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 23.05.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Pierluigi Colli, Università di Pavia, Italien:
A Cahn--Hilliard system with convection and dynamic boundary conditions: Well-posedness and optimal velocity control
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 23.05.2018, 13:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Amal Alphonse, WIAS:
Directional differentiability for elliptic QVIs of obstacle type
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Quasi-variational inequalities (QVIs) are generalisations of variational inequalities where the associated constraint set is no longer explicitly given but it depends on the solution itself. In this talk, we present some work on the directional differentiability of the multi-valued mapping that takes the source term of a QVI onto the set of solutions. This result represents a first step in the study of differential sensitivity of QVIs in infinite dimensions. We also discuss an application to thermoforming and show some numerical experiments.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 23.05.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Markus Reiß/Randolf Altmeyer, Humboldt-Universität zu Berlin:
A nonparametric estimation problem for linear SPDEs
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
(Please find here)

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 22.05.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Tobias Kley, Humboldt-Universität zu Berlin:
Sequential detection of structural changes in irregularly observed data
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
Online surveillance of time series is traditionally done with the aim to identify changes in the marginal distribution under the assumption that the data between change-points is stationary and that new data is observed at constant frequency. In many situations of interest to data analysts, the classical approach is too restrictive to be used unmodified. We propose a unified system for the monitoring of structural changes in streams of data where we use generalised likelihood ratio-type statistics in the sequential testing problem, obtaining the flexibility to account for the various types of changes that are practically relevant (such as, for example, changes in the trend of the mean). The method is applicable to sequences where new observations are allowed to arrive irregularly. Early identification of changes in the trend of financial data can assist to make trading more profitably. In an empirical illustration we apply the procedure to intra-day prices of components of the NASDAQ-100 stock market index. This project is joint work with Piotr Fryzlewicz.

Host
WIAS Berlin
Tuesday, 22.05.2018, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Masahiro Yamamoto, University of Tokyo, Japan:
Inverse problems of determining moving sources in diffusion equations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Thursday, 17.05.2018, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Prof. Ulf Leonhardt, Weizmann Institute of Science, Israel:
Light at the horizon
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Thursday, 17.05.2018, 15:00 (WIAS-406)
FG Stochastische Systeme mit Wechselwirkung
Prof. Dr. Nicolas Petrelis, Université de Nantes, Frankreich:
Scaling limit of the Interacting partially directed self-avoiding walk at criticality
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Further Informations
Seminar FG 5

Host
WIAS Berlin
Thursday, 17.05.2018, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Caroline Geiersbach, Universität Wien, Österreich:
A projected stochastic gradient algorithm for optimization with random elliptic PDE constraints
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Wednesday, 16.05.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Alexander Mielke, WIAS Berlin:
Finding limiting dissipative potentials via EDP convergence
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
A gradient structure for an evolutionary system is a triple, a so-called gradient system, consisting of a state space, a possibly time-dependent energy functional, and a dissipation potential that induces the given evolutionary system. The gradient structure, which is not unique, contains additional thermodynamical information, e.g. on the fluctuations in an associated microscopic model. Considering a family of gradient systems depending on a small parameter, it is natural to ask for the limiting or effective gradient system if the parameter tends to 0. We propose some ideas towards the derivation of the effective gradient structure that are based on De Giorgi's Energy-Dissipation Principle (EDP). We discuss several versions of EDP convergence and show by examples that the theory is flexible enough to allow situations where starting from quadratic dissipation potentials we arrive at effective dissipation potentials that are no longer quadratic.

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 16.05.2018, 13:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Ass.-Prof. Dr. Lothar Banz, Universität Salzburg, Österreich:
A posteriori error estimates for $hp$-dual mixed finite elements by implicit reconstruction
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
A posteriori error estimates are derived for dual mixed methods of $hp$-adaptive finite elements for variational equations as well as for variational inequalities. The error control relies on the use of a special, but never computed, $H^1$-reconstruction of the non-smooth discrete potential. Thus, no post-processing reconstruction (and therewith no additional computation) is needed. Moreover, the use of the discrete potential instead of its reconstruction improves significantly the error estimation in terms of the numerical efficiency indices which are nearly constant and close to one in the numerical experiments. Numerical experiments demonstrate the convergence rates and the efficiency indices of these a posteriori error estimates in $h$- and $hp$-adaptivity.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 16.05.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Moritz Jirak, TU Braunschweig; Martin Wahl, Humboldt-Universität zu Berlin:
Relative perturbation bounds with applications to empirical covariance operators
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
A problem of fundamental importance in quantitative science is to estimate how a perturbation of a covariance operator effects the corresponding eigenvalues and eigenvectors. Due to its importance, this problem has been heavily investigated and discussed in the literature. In this talk, we present general perturbation expansions for a class of symmetric, compact operators. Applied to empirical covariance operators, these expansions allow us to describe how perturbations carry over to eigenvalues and vectors in terms of necessary and sucient conditions, characterising the perturbation transition. We demonstrate the usefulness of these expansions by discussing pca and fpca in various setups, including more exotic cases where the data is assumed to have high persistence in the dependence structure or exhibits (very) heavy tails. This talk is jointly given by Moritz Jirak and Martin Wahl, and divided into two parts

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 15.05.2018, 15:15 (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Dr. Alexander Linke, WIAS Berlin:
On the role of the Helmholtz--Leray projector for a novel pressure-robust discretization theory for the incompressible Navier--Stokes equations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The talk discusses several physical regimes of the incompressible Navier--Stokes equations with respect to the role of the pressure and the role of the Helmholtz--Leray projector in the Navier--Stokes momentum balance. It is emphasized that not the forces in the momentum balance themselves matter for the complexity of the flow field, but their Helmholtz--Leray projector. Since the Helmholtz--Leray projector vanishes for arbitrary gradient fields, a semi norm and a corresponding equivalence class of forces play naturally a major role for the evolution of incompressible flows. Novel pressure-robust mixed finite element methods are designed for an appropriate discrete treatment of this equivalence class of forces. On the contrary, classical, (only) inf-sup stable mixed methods do not care about the existence of such an equivalence class. In order to deliver accurate simulation results, they have to resort to expensive high order ansatz spaces. Finally, the talk will indicate suitable applications for efficient and accurate low-order pressure-robust mixed methods like flows with electrolytes and flows with stagnation points, and it will discuss possible extensions to compressible low-Mach number flows.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 15.05.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Michele Coghi, WIAS Berlin:
Interacting particle systems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Wednesday, 09.05.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Ansgar Jüngel, Technische Universität Wien, Österreich:
Cross-diffusion systems with entropy structure
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
here

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 09.05.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Gitta Kutyniok, Technische Universität Berlin:
Optimal approximation with sparsely connected deep neural networks
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Despite the outstanding success of deep neural networks in real-world applications, most of the related research is empirically driven and a mathematical foundation is almost completely missing. One central task of a neural network is to approximate a function, which for instance encodes a classi cation task. In this talk, we will be concerned with the question, how well a function can be approximated by a neural network with sparse connectivity. Using methods from approximation theory and applied harmonic analysis, we will derive a fundamental lower bound on the sparsity of a neural network. By explicitly constructing neural networks based on certain representation systems, so-called -shearlets, we will then demonstrate that this lower bound can in fact be attained. Finally, we present numerical experiments, which surprisingly show that already the standard backpropagation algorithm generates deep neural networks obeying those optimal approximation rates. This is joint work with H. B?olcskei (ETH Zurich), P. Grohs (Uni Vienna), and P. Petersen (TU Berlin).

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 08.05.2018, 15:15 (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Prof. Marc Timme, Technische Universität Dresden:
Network dynamics as an inverse problem
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Freie Universität Berlin
Tuesday, 08.05.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Valeriy Avanesov, WIAS Berlin:
SDE's, Gaussian processes, clusters, etc.
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Tuesday, 08.05.2018, 13:30 (WIAS-406)
Seminar Materialmodellierung
Dr. Simon Praetorius, Technische Universität Dresden:
From individual motion to collective cell migration
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
here

Host
WIAS Berlin
Monday, 07.05.2018, 13:15 (WIAS-406)
Halbleiterseminar
Dr. Annegret Glitzky, WIAS Berlin:
Thermodynamically consistent models for coupling reaction-diffusion effects in bulk and interfaces
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Further Informations
Halbleiterseminar

Host
WIAS Berlin
Thursday, 03.05.2018, 15:00 (WIAS-ESH)
Seminar Numerische Mathematik
Lukas Feierabend, Universität Duisburg-Essen:
Model development for flowing slurry electrodes in zinc-air batteries
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Slurry or suspension electrodes are gaining a renewed interest for large-scale energy storage technologies due to potentially higher energy densities compared to redox flow batteries [1]. A suspension electrode typically consists of a liquid electrolyte and a solid material, which is chemically active and electrically conducting. For the investigated zinc-air flow batteries employing a suspension electrode, microscopic zinc particles are suspended in an aqueous potassium hydroxide electrolyte [2, 3]. Sedimentation of the metallic zinc is minimized by adding a gelling agent to the electrolyte. The gelling agent and the high particle loading lead to a pseudo-plastic rheological behavior. When the flowable suspension electrode is pumped through the channels of the zinc-air fuel cell setup, the dynamic percolation network within the suspension dictates the active electrode surface area and the maximum discharge power density. Therefore, it is desirable to investigate the influence of the local flow conditions on the particle percolation and consequently the electrochemical performance of the battery cell with adequate simulation methods. In the developed three-dimensional numerical model for the flowing suspension electrode, the complex, non-Newtonian two-phase flow is approximated by a coupling of an Eulerian continuum description for the electrolyte and a discrete, Lagrangian particle description for the motion and interaction of the microscopic particles. The partial differential equations for momentum, species, charge and energy are discretized by the finite volume method and implemented in the OpenFOAM library [4]. The particle motion including multiple simultaneous particle contacts is described with the discrete element method using the LIGGGHTS library [5]. Coupling between the particle and fluid phase is realized with the CFD-DEM method using the CFDEM library [5], where an empirical description accounts for the momentum exchange between the non-Newtonian fluid and the densely distributed particle assemblies. Due to relatively high particle concentrations, the volume displacement by the particles is considered in the electrolyte continuum model. A half-cell model for the anode part of the zinc-air flow battery is implemented, which accounts for the flow characteristics via the described CFD-DEM coupling method. Simultaneously, the charge and species transport is considered according to the classic porous electrode theory by Newman [6] adapted with newer formulations for concentrated electrolytes [7]. In contrast to electrodes with a static porous matrix, the heterogeneous local porosities change temporally depending on the evolving particle distributions. Additionally, the active electrode surface area in each finite volume is dependent on the percolation network from the considered local point to the current collector surface. To estimate the basic parameters for the charge transfer and species transport, a flat-plate electrode with a flowing electrolyte without particles is investigated. In this rather simple reference case, discontinuities of the current density close to the electrode-electrolyte-interface could be observed, which should be inherently excluded due to the model formulations. These inconsistencies will be discussed in detail. Funding by the Federal Ministry for Economic Affairs and Energy (BMWi) of the project â?œZnPLUS -Wiederaufladbare Zink-Luft-Batterien zur Energiespeicherungâ?? and the project â?œZnMobil - Mechanisch und elektrisch wiederaufladbare Zink-Luft-Batterie für automobile Anwendungenâ?? is gratefully acknowledged.

Host
WIAS Berlin
Thursday, 03.05.2018, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Holger Stephan, WIAS Berlin:
One million perrin pseudo primes including a few giants
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Pseudoprimes are integers that are no primes but behave like them in some sense. Suppose we have a theorem like the following: If n is a prime, then statement A(n) holds. In general, the opposite is not true: It may be that A(n) holds, but n is a composite number, a so-called pseudo prime with respect to statement A. Pseudoprimes are interesting if they are very rare, as for instance Perrin's pseudoprimes, the smallest of which is 271441. The talk introduces pseudoprimes which are based on recurrent sequences. In addition, some new numerical results on Perrin's pseudoprimes and a fast algorithm for their calculation are presented.

Host
WIAS Berlin
Wednesday, 25.04.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Nicolai Baldin, University of Cambridge, GB:
Optimal link prediction with matrix logistic regression
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this talk, we will consider the problem of link prediction, based on partial observation of a large network, and on side information associated to its vertices. The generative model is formulated as a matrix logistic regression. The performance of the model is analysed in a high-dimensional regime under a structural assumption. The minimax rate for the Frobenius-norm risk is established and a combinatorial estimator based on the penalised maximum likelihood approach is shown to achieve it. Furthermore, it is shown that this rate cannot be attained by any (randomised) algorithm computable in polynomial time under a computational complexity assumption. (joint work with Q. Berthet)

Further Informations
Forschungsseminar “Mathematische Statistik”

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Monday, 23.04.2018, 14:00 (WIAS-ESH)
Seminar Quantitative Biomedizin
Prof. Sarah Waters, University of Oxford, GB:
Fluid dynamical models for tissue engineering
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Tissue engineers aim to grow tissues in vitro to replace those in the body that have been damaged through age, trauma or disease. A common approach is to seed cells within a scaffold which is then cultured in a bioreactor. The key challenge it to provide the appropriate mechanical and biochemical cellular environment that promotes tissue growth in vitro. Fluid flows have a key role to play in addressing this challenge, as they can provide mechanical cues to cells (e.g. via fluid shear and pressure), and enhance the delivery of nutrients and growth factors (via advection). In this talk, I will explore how mechanistic mathematical models, in combination with state-of-the art experimental studies, can provide quantitative insights into the interplay between the fluid flows and the resulting tissue growth. Quantitative understanding of this interplay offers the exciting potential to manipulate the experimental design (e.g. scaffold porosity, bioreactor operating conditions) to establish defined mechanical cues that enhance the generation of complex 3D tissues in vitro.

Host
WIAS Berlin
Wednesday, 18.04.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Mario Varga, Technische Universität Dresden:
Stochastic unfolding and homogenization
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 18.04.2018, 15:00 (WIAS-406)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Pavel Dvurechensky:
First-order optimization methods with inexact information about the objective function value and its gradient
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
In this talk I will discuss first-order methods with inexact oracle for finite-dimensional optimization. Oracle model of optimization methods assumes that, given a point, the oracle returns some information on the objective function at this point. In the case of first-order optimization methods, this information is the function value and its gradient at this point. I will start with convex problems, inexact oracle defined in the work by O. Devolder, F. Glineur, Yu. Nesterov, Math. Prog., 2014, and convergence rates for gradient descent and accelerated gradient descent in this case. I will also describe an extension for non-convex problems. Then I will discuss some ideas on how these methods potentially can be extended and applied for infinite-dimensional problems. If time allows, I will cover other optimization problems and methods, which I work with. Among others, optimal transport problem and an accelerated gradient descent for its solution, randomized optimization methods, such as random coordinate descent and random derivative-free method, variational inequalities, saddle-point problems and first-order methods for their solution.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 18.04.2018, 13:30 (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Paul Deuring, Université du Littoral “Côte d'Opale”, Frankreich:
Stabilitäts- und Fehlerschranken einer FE-FV-Diskretisierung von Konvektions-Diffusionsgleichung: exponentielle Abhängigkeit von Parametern
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Wir betrachten eine FE-FV-Methode zur Bestimmung von Näherungslösungen von Konvektions-Diffusionsgleichungen. Bei dieser Methode wird der Diffusionsterm mittels Crouzeix-Raviart-Elementen diskretisiert, während man den Konvektionsterm durch baryzentrische finite Volumen approximiert. Wir gehen der Frage nach, ob sich Stabilitäts- und Fehlerschranken für diese Methode finden lassen, die von keiner Größe exponentiell abhängen, insbesondere nicht vom Kehrwert des Diffusionskoeffizienten.

Host
WIAS Berlin
Wednesday, 18.04.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Alexandra Suvorikova, WIAS Berlin:
Gaussian process forecast with multidimensional distributional input
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In this work, we focus on forecasting a Gaussian process indexed by probability distributions. We introduce a family of positive definite kernels constructed with the use of optimal transportation distance and provide their probabilistic understanding. The technique allows to forecast efficiently Gaussian processes, which opens new perspective in Gaussian process modelling.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 17.04.2018, 15:15 (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Misha Zaks, Universität Potsdam:
Collective dynamics of globally coupled excitable units
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 17.04.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Peter Friz, WIAS und TU Berlin:
“Algebraic statistics meets rough paths”, joint work with C. Armendola and B. Sturmfels
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
The signature of a parametric curve is a sequence of tensors whose entries are iterated integrals. This construction is central to the theory of rough paths in stochastic analysis. It is here examined through the lens of algebraic geometry. We introduce varieties of signature tensors for both deterministic and random paths. For the former, we focus on piecewise linear paths, on polynomial paths, and on varieties derived from free nilpotent Lie groups. For the latter, we focus on Brownian motion and its mixtures.

Host
WIAS Berlin
Thursday, 12.04.2018, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Michael J. Neilan, University of Pittsburgh, USA:
Inf-sup stable Stokes pairs on barycentric refinements producing divergence-free approximations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We construct several stable finite element pairs for the Stokes problem on barycentric refinements in arbitrary dimensions and for any polynomial degree. A key feature of the spaces is that the divergence maps the discrete velocity space onto the discrete pressure space; thus, when applied to models of incompressible flows, the pairs yield divergence-free velocity approximations. The key ingredients to prove these results are local inf-sup stability estimates and a modification of Bernardi-Raugel bubble functions. This is joint work with Johnny Guzman.

Host
WIAS Berlin
Tuesday, 10.04.2018, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Ozan Öktem, KTH Royal Institute of Technology, Schweden:
Recent advances in using machine learning for image reconstruction
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
The talk will outline recent approaches for using (deep) convolutional neural networks to solve a wide range of inverse problems, such as image reconstruction in medical tomography. A key element is to use a neural network architecture for reconstruction that includes physics based models that describe how data is generated as well as its statistical properties. Another is the possibility to integrate complex task related a priori information and elements of decision making into the reconstruction procedure. The resulting approach outperforms current state-of-the-art in terms of 'quality', computational speed and there is no need to manually set parameters as with variational methods. Furthermore, the amount of training data and network size can be kept surprisingly small.

Host
WIAS Berlin
Tuesday, 27.03.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Elena Ivanova, AOK:
Community detection on evolving graphs
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We consider algorithms for detecting the cluster structure in a continual evolving decentralised graph with large number of vertices. As a model for the graph we use the evolving stochastic block model, where only the set of edges changes over time. Algorithms with low upper error bounds will be presented.

Host
WIAS Berlin
Tuesday, 27.03.2018, 13:30 (WIAS-406)
Seminar Materialmodellierung
Dr. Esteban Meca, University of Cordoba, Spanien:
Localized instabilities in phase-changing systems: The effect of elasticity
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Tuesday, 20.03.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Benjamin Stemper, WIAS und TU Berlin:
Advances in calibrating rough stochastic volatility models via neural networks
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We consider rough stochastic volatility models where the driving noise of volatility has fractional scaling, in the “rough" regime of Hurst parameter H < 1/2. This regime recently attracted a lot of attention both from the statistical and option pricing point of view for its ability to better fit market data than conventional stochastic volatility models like Heston. A drawback of the new regime is that efficient pricing schemes are yet unknown, making calibration - where repetitive evaluation of the pricing functional is required - prohibitively expensive. In our last talk, we envisioned a calibration scheme using deep learning where the expensive pricing functional is learned by an artificial neural network. In this talk, we report on our progress and share some challenges encountered on the way. Work in progress, joint with Christian Bayer.

Host
WIAS Berlin
Thursday, 08.03.2018, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Sabrina Pickartz:
Optical soliton control by interactions with dispersive waves
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 07.03.2018, 14:00 (WIAS-ESH)
Seminar Materialmodellierung
Dr. Matthias Liero, WIAS Berlin:
Modeling and simulation of charge transport in organic semiconductors via kinetic and drift-diffusion models
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The use of organic materials in electronic applications such as displays, photovoltaics, lighting, or transistors, has seen an substantial increase in the last decade. This is mainly due to the lower production cost, sustainability, and flexibility. Moreover, the toolbox of organic chemistry opens an enormous potential for new device concepts. In contrast to classical semiconductors such as silicon or gallium-arsenide, charge-transport in organic materials happens via temperature activated hopping transport of electrons or holes between adjacent molecules. Here, the crucial feature is the random alignment of the molecule, which leads to a disordered system with Gaussian distributed energy levels. A common approach to simulate the transport of charge carriers in organic materials is based on a master equation description of the hopping transport and kinetic Monte-Carlo methods. However, the computational costs of this approach are typically very high and the treatment of complicated multi-dimensional device structures is very challenging. Moreover, the inclusion of multi-physics effects such as heat flow is out of scope. The latter, in particular, is of high importance as organic devices show a strong interplay between electrical current and heat flow. Here, drift-diffusion models provide an immense advantage. In this talk, we give an overview over the two modeling approaches and give an outlook to future challenges.

Host
WIAS Berlin
Tuesday, 06.03.2018, 15:15 (WIAS-ESH)
Seminar Laserdynamik
Dr. Otti D'Huys, Aston University, GB:
Dynamics of delay-coupled networks with time-varying connections
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Seminar Laserdynamik

Host
WIAS Berlin
Thursday, 01.03.2018, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Franco Dassi, Politecnico di Milano, Italien:
Recent advancements of the Virtual Element Method in 3D
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The Virtual Element Method is a novel way to discretize a partial differential equation. It avoids the explicit integration of shape functions and introduces an innovative construction of the stiffness matrix so that it acquires very interesting properties and advantages. One among them is the possibility to apply the VEM to general polygonal/polyhedral domain decomposition, also characterized by non-conforming and non-convex elements.
In this talk we focus on the definition/construction of the Virtual Element functional spaces in three dimensions and how apply this new strategy to set a standard Laplacian problem in 3D.
Finally we test the method to show its robustness with respect to element distortion and the polynomial approximation degree $k$. Then, we move to more involved cases: convection-diffusion-reaction problems with variable coefficients and magnetostatic Maxwell equations.

Host
WIAS Berlin
Wednesday, 28.02.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Moritz Egert, Université de Paris-Sud, Frankreich:
How half-order time derivatives help us to better understand parabolic equations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In the study of parabolic problems, the prototypes of which are related to the heat operator ∂ t - Δ, an old but often overlooked approach is to decompose the time-derivative into two fractional derivatives of order 1/2. Depending on ones preferences, this can either be considered 'natural' since it brings back the parabolic homogeneity, for instance when the elliptic part is interpreted in the weak sense via a sesquilinear form, or rather 'peculiar' as it introduces non-local operators in the context of local equations.

In this talk I shall touch on some recent results that used half-order time-derivatives in a crucial way, placing an emphasis on my own contributions obtained with several collaborators. This includes improvements of time-regularity for solutions to parabolic systems with measurable coefficients of second as well as fractional order, non-autonomous maximal regularity for the related initial value problem, a new approach to parabolic boundary value problems, and the solution of the parabolic Kato square root problem.

Host
Humboldt-Universität zu Berlin
WIAS Berlin

Monday, 26.02.2018, 14:00 (WIAS-ESH)
Seminar Laserdynamik
Dr. Karin Mora, Universität Paderborn:
Says Newton to Hopf and Hogkin--Huxley: Vibrating! Impacting! Exciting!
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Wednesday, 21.02.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar) & Seminar Materialmodellierung
Dr. Marco Morandotti, TU München:
Dimension reduction in the context of structured deformations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The theory of structured deformations shows good potential to deal with mechanical problems where multiple scales and fractures are present. Math- ematically, it amounts to relaxing a given energy functional and to show also the relaxed one has an integral representation. In this seminar, I will focus on a problem for thin objects: the derivation of a 2D relaxed energy via dimension reduction from a 3D energy, incorporat- ing structured deformations in the relaxation procedure. I will discuss the two-step relaxation (first dimension reduction, then structured deformations and vice versa) and I will compare it with another result in which the two relaxation procedures are carried out simultaneously. An explicit example for purely interfacial initial energies will complete the presentation. These results have been obtained in collaboration with G. Carita, J. Matias, and D.R. Owen.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 20.02.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Pavel Dvurechensky, WIAS Berlin:
Faster algorithms for (regularized) optimal transport
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We propose an alternative to ubiquitous Sinkhorn algorithm for solving regularized optimal transport problem. Our approach is based on accelerated gradient descent applied to the dual problem and allows to solve not only entropy-regularized optimal transport problem, but also squared-2-norm-regularized OT problem, which produces sparser optimal transport plan approximation. We prove that, our algorithm, allows to solve non-regularized optimal transport problem with support size $p$ up to accuracy $epsilon$ in $Oleft(minleftfracp^9/4epsilon, fracp^2epsilon^2 rightright)$ arithmetic operations, which is better than state-of-the-art result $Oleft(fracp^2epsilon^3right)$.

Host
WIAS Berlin
Thursday, 15.02.2018, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Jan-Philipp Koester, Ferdinand-Braun-Institut Berlin:
Simulation of pulsed laser diodes driven far above threshold with WIAS--BALaser and comparison with experiment
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Wednesday, 14.02.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Wolfgang Dreyer, WIAS Berlin:
Modeling of non-Newtonian fluids without material frame indifference
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In the fifties the modeling of non-Newtonian fluids initiated the search for invariant time derivatives with respect to certain space time transformations. Then Euclidean Transformations were selected to establish the Principle of Material Frame Indifference and Nth Grade Fluids. However, the two concepts lead to serious inconsistencies between thermodynamic consistent models and experimental observations. In 1986 the subject has been resolved in a remarkable paper by I. Müller and K. Wilmanski. We discuss alternative non-Newtonian fluids models of Maxwell-type and of Balance-type within Continuum Thermodynamics as it was laid down by D. Bothe and W. Dreyer.

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Tuesday, 13.02.2018, 15:15 (FU-140)
Oberseminar Nonlinear Dynamics
Marek Fila, Comenius University Bratislava, Slovakia:
A Gagliardo--Nirenberg-type inequality and its applications to decay estimates for solutions of a degenerate parabolic equation
more ... Location
Freie Universität Berlin, Arnimallee 7, 14195 Berlin, Hinterhaus, Raum: 140

Abstract
We discuss a Gagliardo-Nirenberg-type inequality for functions with fast decay. We use this inequality to derive upper bounds for the decay rates of solutions of a degenerate parabolic equation. Moreover, we show that these upper bounds, hence also the Gagliardo-Nirenberg-type inequality, are sharp in an appropriate sense. The talk will consist of two parts - an introduction of one hour and a presentation of proofs of also one hour.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 13.02.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Martin Redmann, MPI Magdeburg:
Solving parabolic rough partial differential equations using regression
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
In this talk, we discuss the numerical approximation of parabolic partial differential equations (PDEs) driven by a rough path which, e.g., can be the lift of a path of a fractional Brownian motion. By using the Feynman-Kac formula, the solution can be represented as the expected value of a functional of the corresponding hybrid Stratonovich-rough differential equation. A time-discretisation of this equation and a Monte Carlo regression in the spatial variable lead to an approximation of the solution to the rough PDE. We analyse the regression error and provide several numerical experiments to illustrate the performance of our method.

Host
WIAS Berlin
Wednesday, 07.02.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Gitta Kutyniok, Technische Universität Berlin:
Optimal approximation with sparsely connected deep neural networks
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Despite the outstanding success of deep neural networks in real-world applications, most of the related research is empirically driven and a mathematical foundation is almost completely missing. One central task of a neural network is to approximate a function, which for instance encodes a classi cation task. In this talk, we will be concerned with the question, how weural network with sparse connectivity. Using methods from approximation theory and applied harmonic analysis, we will derive a fundamental lower bound on the sparsity of a neural network. By explicitly constructing neural networks based on certain representation systems, so-called -shearlets, we will then demonstrate that this lower bound can in fact be attained. Finally, we present numerical experiments, which surprisingly show that already the standard backpropagation algorithm generates deep neural networks obeying those optimal approximation rates. This is joint work with H. B?olcskei (ETH Zurich), P. Grohs (Uni Vienna), and P. Petersen (TU Berlin).

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 06.02.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Mario Maurelli, WIAS Berlin:
McKean-Vlasov SDEs with irregular drift: large deviations for particle approximation
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
McKean-Vlasov SDEs are SDEs where the coefficients depend on the law of the solution to the SDE. Their interest is in the links with nonlinear PDEs on one side (the SDE-related Fokker-Planck equation is nonlinear) and with interacting particles on the other side: the McKean-Vlasov SDE can be approximated by a system of weakly coupled SDEs. In this talk we consider McKean-Vlasov SDEs with irregular drift: though well-posedness for this SDE is not known, we show a large deviation principle for the corresponding interacting particle system. This implies, in particular, that any limit point of the particle system solves the McKean-Vlasov SDE. The proof combines rough paths techniques and an extended Varadhan lemma. This is a joint work with Thomas Holding.

Host
WIAS Berlin
Friday, 02.02.2018, 14:30 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Thomas Ruf, Universität Augsburg:
On a variational approach to the nonlinear wave equation
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In a 2012 paper, E. Serra and P. Tilli proved a conjecture by E. de Giorgi stating that global weak solutions of wave equations such as w” - Delta w + wlwl^(p-2) = 0 on R^+ times  R^d can be obtained as limits of minimizers of suitable variational functionals. We generalize this proof to equations of the form w” - Delta w + f_w (t; x; w) = 0 with p-growth conditions on f, also replacing R^d by an arbitrary open set O subset R^d and suitable boundary conditions.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Thursday, 01.02.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Alexander Gasnikov, Moscow Institute of Physics and Technology, Russia:
Recent developments of accelerated gradient descent, part II
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Thursday, 01.02.2018, 15:00 (WIAS-ESH)
Seminar Numerische Mathematik
Laura Blank, WIAS Berlin:
Towards a simple and robust finite element method for the numerical simulation of porous media flow
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The topic of this talk is the numerical simulation of flow through porous media based on the Brinkman model as a unified framework that allows the transit between Darcy and Stokes problems. Therefore an unconditionally stable low order finite element approach, which is robust with respect to the physical parameters, is proposed. This approach is based on the combination of stabilized equal order finite elements with a non-symmetric (penalty-free) Nitsche method for the weak imposition of essential boundary conditions. Focusing on the two-dimensional case, optimal a priori error estimates in a mesh-dependent norm, which allows to extend the results also to the Stokes and Darcy limits, are obtained.

Host
WIAS Berlin
Thursday, 01.02.2018, 14:00 (WIAS-ESH)
Seminar Quantitative Biomedizin
Dr. Edoardo Sinibaldi, Italian Institute of Technology:
Selected modeling approaches for biomedical applications and biorobotics tools
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The effective deployment of theranostic agents (including drugs), smart?materials?based systems and interfaces to target regions (prospectively) in the human body where to perform the sought actions also requires theoretical models and physical tools. Modeling the delivery and the (remote) actuation/stimulation of the aforementioned agents/systems/interfaces, in particular, permits to compensate for experimental conditions hard to characterize and to better interpret the experimental results, thus paving the way for quantitative therapy design and control. This is complemented by model-based design of related experimental devices, flexible tools able to safely navigate anatomical pathways, and miniaturized effectors. In this talk I will firstly present the solution to an inverse problem, namely to determine the velocity profile in a vessel cross-section starting from the flow-rate, which is relevant to pulsatile biological flows (blood and cerebro-spinal fluid flows), with application, e.g., to magnetic particle targeting. Then I will address numerical models for intrathecal drug-delivery (drug infusion in the cerebro-spinal fluid, also accounting for transport to the spinal cord) and simplified analytical models for intra-tissue drug-delivery (particle transport into porous/poroelastic media, with application to intratumoral thermotherapy). Finally, I will overview model-based approaches for: piezoelectric nanoparticle-mediated cell stimulation; real-scale physical models of the blood-brain barrier; flexible biorobotics probes; miniature (bioinspired) actuators and effectors.

Host
WIAS Berlin
Wednesday, 31.01.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Alexander Linke, WIAS Berlin:
On the role of the Helmholtz--Leray projector for a novel pressure-robust discretization theory for the incompressible Navier--Stokes equations
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The talk discusses several physical regimes of the incompressible Navier--Stokes equations with respect to the role of the pressure and the role of the Helmholtz--Leray projector in the Navier--Stokes momentum balance. It is emphasized that not the forces in the momentum balance themselves matter for the complexity of the flow field, but their Helmholtz--Leray projector. Since the Helmholtz--Leray projector vanishes for arbitrary gradient fields, a semi norm and a corresponding equivalence class of forces play naturally a major role for the evolution of incompressible flows. Novel pressure-robust mixed finite element methods are designed for an appropriate discrete treatment of this equivalence class of forces. On the contrary, classical mixed methods do not care about the existence of such an equivalence class. In order to deliver accurate simulation results, they have to resort to expensive high order ansatz spaces. Finally, the talk will indicate suitable applications for efficient and accurate low-order pressure-robust mixed methods like flows with electrolytes, flows with stagnation points and flows with a dominant Coriolis force.

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 31.01.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Elisabeth Gassiat, Universite Paris-Sud:
Estimation of the proportion of explained variation in high dimensions
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The estimation of the heritability of a phenotypic trait based on genetic data may be set as the estimation of the proportion of explained variation in high dimensional linear models. I will be interested in understanding the impact of: not knowing the sparsity of the regression parameter, not knowing the variance matrix of the covariates on minimax estimation of heritability. In the situation where the variance of the design is known, I will present an estimation procedure that adapts to unknown sparsity. When the variance of the design is unknown and no prior estimator of it is available, I will show that consistent estimation of heritability is impossible. (Joint work with N. Verzelen, and PHD thesis of A. Bonnet).

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Wednesday, 24.01.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Hagen Neidhardt, WIAS Berlin:
On the spectral shift function: Recent results
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 24.01.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Andreas Maurer, München:
Concentration for functions of bounded interaction
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Some multivariate functions have the property that their variation in any argument does not change too much when another argument is modi ed. The talk will give some examples and concentrates on the random variable W obtained by applying such a function to a vector of independent variables. Functions with weakly interacting arguments share some important properties with sums: the expectation of W can be estimated by a version of Bernstein's inequality and its variance can be tightly estimated in terms of an iid sample, which has only one datum more than the function has arguments. There is also a version of the central limit theorem.

Host
Universität Potsdam
Humboldt-Universität zu Berlin
WIAS Berlin
Tuesday, 23.01.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Paolo Pigato, WIAS Berlin:
The exploding skew of implied volatility
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
We show how a simple local volatility model, with volatility function discontinuous at-the-money, is capable of reproducing the explosion of the implied volatility skew, observed on empirical data. We use such local volatility as a leverage function, in a stochastic-local volatility model, obtaining good fits of the whole implied volatility surface on empirical observations.

Host
WIAS Berlin
Tuesday, 23.01.2018, 14:00 (WIAS-406)
Seminar Materialmodellierung
Prof. Jan Giesselmann, RWTH Aachen:
Modelling error estimates and model adaptation in compressible flows
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
tba

Host
WIAS Berlin
Friday, 19.01.2018, 14:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Prof. Dr. Sören Bartels, Albert-Ludwigs-Universität Freiburg:
Finite element methods for nonsmooth problems and application to a problem in optimal insulation
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Nonsmooth problems arise in the mathematical modeling of contact and obstacle problems, the description of plastic material behavior, and mathematical image processing. The unknown functions are typically characterized as minimizers of nondifferentiable functionals. Numerical schemes approximately solve these problems either via duality methods or classically by making use of appropriate regularizations. In the talk we discuss the discretization and iterative solution of a model problem defined on functions of bounded variation. The numerical analysis of finite element discretizations leads to reduced convergence rates which can be improved using adaptive mesh refinement. Suitable iterative solution procedures are ADMM schemes, for which we propose an automatic step size adjustment strategy, and gradient flows, for which we demonstrate the unconditional stability of a semi-implicit time discretization. The methods are applicable in the numerical determination of optimal insulating films for heat conducting bodies. Below a critical value of available insulation mass an unexpected break of symmetry occurs.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Thursday, 18.01.2018, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Wolfgang Dreyer, WIAS Berlin:
Modeling of non-Newtonian fluids without material frame indifference
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
In the fifties the modeling of non-Newtonian fluids initiated the search for invariant time derivatives with respect to certain space time transformations. Then Euclidean Transformations were selected to establish the Principle of Material Frame Indifference and moreover, the introduction of Nth grade fluids. However, the two concepts lead to serious inconsistencies in both thermodynamic modeling and experimental observations. In 1986 the subject has been resolved in a remarkable paper by I. Müller and K. Wilmanski. In this lecture we show how an appropriate model of non-Newtonian fluids may be incorporated in Continuum Thermodynamics as it was laid down by D. Bothe and W. Dreyer.

Host
WIAS Berlin
Wednesday, 17.01.2018, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Jürgen Sprekels, WIAS Berlin:
Cahn--Hilliard systems with general fractional-order operators
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 17.01.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Anthony Nouy, École Centrale de Nantes, Frankreich:
Learning high-dimensional functions with tree tensor networks
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Tensor methods are among the most prominent tools for the approximation of high- dimensional functions. Such approximation problems naturally arise in statistical learning, stochastic analysis and uncertainty quanti cation. In many practical situations, the approximation of high- dimensional functions is made computationally tractable by using rank-structured approximations. In this talk, we give an introduction to tree-based (hierarchical) tensor formats, which can be interpreted as deep neural networks with particular architectures. Then we present adaptive algorithms for the approximation in these formats using statistical methods.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 16.01.2018, 15:15 (FU-140)
Oberseminar Nonlinear Dynamics
Jia-Yuan Dai, Freie Universität Berlin:
Existence of local solutions of Gowdy spacetimes
more ... Location
Freie Universität Berlin, Arnimallee 7, 14195 Berlin, Hinterhaus, Raum: 140

Abstract
We consider a class of Gowdy spacetimes that reduces the Einstein's field equation to a system of two semilinear wave equations, by assuming a universe without matter, in which the gravitational wave fronts repeat in space and are mutually parallel. To prove the existence of local solutions of the system, we add a linear perturbation and seek periodic-in-space solutions. The idea of the proof is to apply the Lyapunov-Schmidt reduction. We will solve a related small-divisor problem and discuss how to design the correct functional setting that fits to the nonlinearity. This ongoing research is a joint work with Dr. Hannes Stuke.

Host
Freie Universität Berlin
WIAS Berlin
Tuesday, 16.01.2018, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Alexander Gasnikov, Moscow Institute of Physics and Technology, Russia:
Recent developments of accelerated gradient descent, part I
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Tuesday, 16.01.2018, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Ingo Bremer, WIAS Berlin:
Introduction to a framework for nonlinear optimization with adaptation to probabilistic constraints
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Thursday, 11.01.2018, 16:00 (WIAS-ESH)
Seminar Laserdynamik
Prof. Andrey L. Shilnikov, Neuroscience Institute, Georgia State University, USA:
Torus bifurcations in slow fast systems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Thursday, 11.01.2018, 15:00 (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Amer Rasheed, Lahore University of Management Sciences, Pakistan:
Influence of magnetic field on dendrites during solidification of binary mixtures
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
A phase field model has been proposed which incorporates convection and magnetic field in an isothermal environment. A numerical scheme is proposed and numerical analysis of model in two-dimensional geometry is performed. The numerical stability and error analysis of this approximation scheme which is based on mixed finite-element method are performed. An application of a nickel-copper binary alloy is considered. Influence of various magnetic fields on the dendrites during the solidification process has been discussed.

Host
WIAS Berlin
Thursday, 11.01.2018, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Gert Lube, Georg-August-Universität Göttingen:
Why exactly divergence-free H(div)-conforming FEM for transient incompressible flows?
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Recent results show that one can reconstruct classical inf-sup stable H1-conforming FEM for incompressible flow problems to be pressure-robust. Exactly divergence-free FEM are pressure-robust per construction. In particular, exactly divergence-free H(div)-conforming FEM combine excellent stability and conservation properties with minimal stabilization. We show that the numerical analysis of such methods allows to separate the (static) linear Stokes regime and the nonlinear dynamic regime. Semi-robust error estimates w.r.t. the Reynolds number will be given. In a hybridized form, H(div)-conforming FEM are suitable for large scale computation. Applications of the approach to two- and three-dimensional problems of vortex dynamics will be presented for high Reynolds numbers.

Host
WIAS Berlin
Thursday, 11.01.2018, 10:00 (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Tobias Kluth, Universität Bremen:
Model-based magnetic particle imaging
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Magnetic particle imaging (MPI) is a tracer-based imaging modality developed to detect the concentration of superparamagnetic iron oxide nanoparticles. It is highly sensitive to the nanoparticle's nonlinear response to a dynamic applied magnetic field. Model-based reconstruction techniques are still not able to reach the quality of data-based approaches in which the linear system function is determined by a time-consuming measurement process. Possible reasons include the relaxation behavior of nanoparticles in fast changing magnetic fields. However, the equilibrium model described by the Langevin function is still used to predict the system behavior. In this context we discuss the ill-posedness of the imaging problem. We further focus on the model-based MPI reconstruction problem incorporating deviations in the forward operator. This is illustrated by initial results from real data using a regularized total least squares approach.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 10.01.2018, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Antoine Chambaz, Universität Paris Descartes, Frankreich:
An introduction to targeted learning
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Coined by Mark van der Laan and Dan Rubin in 2006, targeted learning is a general approach to learning from data that reconciles machine learning and statistical inference. On the one hand, machine learning" refers to the estimation of in nite-dimensional features of the law of the data, P, for instance a regression function. Machine learning algorithms are versatile, and produce (possibly highly) data-adaptive estimators. Driven by the need to make accurate predictions, they do not care so much about the assessment of prediction uncertainty. On the other hand, statistical inference" refers to the estimation of nite-dimensional parameters of P, for instance a measure of association with a causal interpretation. It focuses on the construction of con dence regions or the development of hypotheses tests. Emphasis is placed on robustness (guaranteeing that one goes to the truth even under mild and reasonable assumptions on P), eciency (trying to draw as much information from the data as possible), and controlling the asymptotic levels or type I errors.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Thursday, 21.12.2017, 14:15 (WIAS-HVP-4.13)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Guozhi Dong, Humboldt-Universität zu Berlin:
Regularization methods and nonlinear PDEs for solving inverse and imaging problems
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 4. Etage, Raum: 4.13

Abstract
In this talk, I will present some recent developments on regularization methods in image sciences. I shall show a tiny background and also mention some of the state of the art in this area. The focus will be then to discuss some non-convex regularization models for specific problems which suffer from displacement errors. The non-convex energy functionals reveal to have tight connections to some nonlinear (geometric) partial differential equations, e.g. mean curvature flows. Finally, I will show some numerical results with some discussions on the algorithms.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
Humboldt-Universität zu Berlin
WIAS Berlin
December 18 – 19, 2017 (WIAS-ESH)
Workshop/Konferenz: Dynamics of novel mode-locked and frequency-swept lasers
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Thursday, 14.12.2017, 14:00 (WIAS-ESH)
Seminar Materialmodellierung
Bartlomiej Matejczyk, University of Warwick, GB:
Macroscopic models for ion transport in nanoscale pores
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
During this talk, we discuss ionic transport through confined geometries. Our problem concerns modelling ionic flow through nanopores and ion channels. We present different methods of engineering the pores together with its characteristics. Next, we comment on the challenges in simulating the flow efficiently. In the second part, we present a derivation of 1D asymptotic reduction of the Poisson-Nernst-Planck model applied to long and narrow pores. We discuss numerical schema and compare the results the solution of the two-dimensional system of equations.

Host
WIAS Berlin
Wednesday, 13.12.2017, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Joachim Rehberg, WIAS Berlin:
On maximal parabolic regularity for non-autonomous parabolic operators, Part 2
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 13.12.2017, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Fabian Dunker, University of Canterbury, Christchurch, New Zealand:
Multiscale tests for shape constraints in linear random coefficient models
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
A popular way to model unobserved heterogeneity is the linear random coecient mo- del Y i = i;1Xi;1 + i;2Xi;2 + ::: + i;dXi;d. We assume that the observations (Xi; Yi); i = 1; :::; n, are i.i.d. where Xi = (Xi;1; :::;Xi;d) is a d-dimensional vector of regressors. The random coecients i = ( i;1; :::; i;d); i = 1; :::; n, are unobserved i.i.d. realizations of an unknown d-dimensional distribu- tion with density f independent of Xi. We propose and analyze a nonparametric multi-scale test for shape constraints of the random coecient density f . In particular we are interested in con dence sets for slopes and modes of the density. The test uses the connection between the model and the d-dimensional Radon transform and is based on Gaussian approximation of empirical processes. This is a joint work with K. Eckle, K. Proksch, and J. Schmidt-Hieber.

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 12.12.2017, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Franz Besold, Humboldt-Universität zu Berlin:
Persistence diagrams
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
In this talk we introduce persistence diagrams. These can be used as a tool to infer topological information from noisy data. To do that, we review simplicial and singular homology. Persistence diagrams have originally be defined for functions on topological spaces, but can be more generally defined using persistence modules. Stability results ensuring that close data sets have close persistence diagrams show that persistence diagrams are well-suited to deal with real-life data. Applications include data smoothing or recovering topological features of manifolds from a sampled point cloud.

Host
WIAS Berlin
Monday, 11.12.2017, 14:00 (WIAS-ESH)
INSTITUTSKOLLOQUIUM
Jürgen Borchardt/Peter Mathé, WIAS:
Der block-orientierte Prozesssimulator BOP: Was er kann und was er tat.
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
BOP dient zur Prozesssimulation, insbesondere zum Lösen großer DAE Systeme (differential-alegebraic systems). Über viele Jahre hinweg wurden mit BOP Simulationen von Gas/Öl-Turbinen vorgenommen, in der Kooperation mit Alstom, Switzerland. Um eine derartige praxisbezogene Zusammenarbeit langfristig zu gestalten, wurden die Fähigkeiten des solvers weiterentwickelt. Peripher wurden verschiedene Verfahren implementiert, insbesondere zur deterministischen und Bayes'schen Optimierung. Die Simulationsergebnisse können vielfältig visualisiert werden. Im Vortrag werden diese Entwicklungen vorgestellt. Weiterhin wird die erfolgreiche Zusammenarbeit mit dem Kooperationspartner im Rückblick skizziert.

Host
WIAS Berlin
Wednesday, 06.12.2017, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Joachim Rehberg, WIAS Berlin:
On maximal parabolic regularity for non-autonomous parabolic operators, Part 1
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 06.12.2017, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Alexander Meister, Universität Rostock:
Nonparametric density estimation for intentionally corrupted functional data
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
We consider statistical models where, in order to satisfy privacy constraints, functional data are artificially contaminated by independent Wiener processes. We show that the corrupted observations have a Wiener density, which determines the distribution of the original functional random variables uniquely. We construct a nonparametric estimator of the functional density and study its asymptotic properties. We provide an upper bound on its mean integrated squared error which yields polynomial convergence rates, and we establish lower bounds on the minimax convergence rates which are close to the rates attained by our estimator. Our estimator requires the choice of a basis and of two smoothing parameters. We propose data-driven ways of choosing them and prove that the asymptotic quality of our estimator is not significantly affected by the empirical parameter selection. We apply our technique to a classification problem of real data and provide some numerical results. This talk is based on a joint work with A. Delaigle (University of Melbourne).

Host
WIAS Berlin
Tuesday, 05.12.2017, 15:00 (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Maxim Panov, Skolkovo Institute of Science and Technology, Moskow, Russia:
Consistent estimation of mixed memberships with successive projections
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
In this talk, we consider several types of accelerated randomized gradient methods: random directional search, random coordinate descent, randomized zero-order method. Using the concept of inexact oracle, we present a generic theorem on the convergence rate for all the three methods. Despite their random nature, these methods have complexity with the same dependence on the desired accuracy of the solution as deterministic accelerated gradient method. Joint work with A. Gasnikov and A. Tiurin.

Host
WIAS Berlin
Tuesday, 05.12.2017, 10:00 (WIAS-ESH)
Seminar Laserdynamik
Prof. Raimondas Čiegis, Vilnius Gediminas Technichal University, Litauen:
Numerical simulation of nonlocal delayed feedback controller for some new smart bioreactors
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The new nonlocal delayed feedback controller is used to control the production of drugs in a simple bioreactor. This bioreactor is based on the enzymatic conversion of substrate into the required product. The dynamics of this device is described by a system of two nonstationary nonlinear diffusion-reaction equations. The control loop defines the changes of the substrate concentration delivered into the bioreactor at the external boundary of the bioreactor depending on the difference of measurements of the produced drug delivered into the body and the flux of the drug prescribed by a doctorin accordance with the therapeutic protocol. The system of PDEs is solved by using the finite difference method, the control loop parameters are defined from the analysis of stationary linearized equations. The stability of the algorithm for the inverse boundary condition is investigated. Results of computational experiments are presented and analyzed.

Host
WIAS Berlin
Thursday, 30.11.2017, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Sabrina Pickartz, WIAS Berlin:
All-optical control of fiber-solitons
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Thursday, 30.11.2017, 14:00 (WIAS-ESH)
Seminar Numerische Mathematik
PD Dr. Michal Pavelka, Charles University, Tschechische Republik:
42=GENERIC - Unified Hamiltonian description of solids and fluids
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
GENERIC (General equation for non-equilibrium reversible-irreversible coupling) provides evolution equations for many physical theories: fluid mechanics, kinetic theory, mechanics of solids, plasticity, non-Newtonian fluid mechanics, electromagnetic field coupled with matter, dynamics of mixtures, etc. The equations consist of a reversible Hamiltonian part, generated by a Poisson bracket and energy, and by an irreversible gradient part, generated by a dissipation potential and entropy. The purpose of this talk is to provide introduction into GENERIC and discuss some specific applications.

Host
WIAS Berlin
Wednesday, 29.11.2017, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Pavel Gurevich, Freie Universität Berlin:
Rattling in hysteretic reaction-diffusion systems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Tuesday, 28.11.2017, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Christian Löbbert, RWTH Aachen:
Parallel arithmetic for distributed tensors in the HT-format
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Monday, 27.11.2017, 14:00 (WIAS-ESH)
Dr. Anne Höner, Forschungsverbund Berlin e.V.:
Funding opportunities for WIAS Researchers within the European Framework Programme Horizon 2020
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Monday, 27.11.2017, 09:30 (WIAS-HVP-4.13)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Andrea Ceretani, Humboldt-Universität zu Berlin:
Anomalous diffusion with free boundaries
more ... Location
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 4. Etage, Raum: 4.13

Abstract
Recently, time fractional Stefan-like problems have been used to model anomalous diffusion with free boundaries and long memory retention. Nevertheless, the physical foundations of this usage is still unclear. We present a model for acid water neutralization with anomalous and fast diffusion. Though this problem presents short memory retention, it is a first step in deriving mathematical models for anomalous diffusion under memory effects based on commonly accepted physical laws. The problem consists in the neutralization of an acid solution in which the hydrogen ions are transported according to Cattaneo's diffusion, and we consider the specific case of a marble slab reacting with a sulphuric acid solution in a one-dimensional geometry. The mathematical problem is reduced to a hyperbolic free boundary problem where the consumption of the slab is described by a nonlinear differential equation. We prove global well-posedness and present some numerical simulations.

Further Informations
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Friday, 24.11.2017, 14:00 (WIAS-ESH)
Halbleit