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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, 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
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
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
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