Oberseminar Stochastik

Alice Callegaro, JGU

Survival and complete convergence for a branching annihilating random walk

Branching systems with competition are interacting particle systems which have gained popularity as models for the reproduction of a spatial population with limited environmental resources. We study a branching annihilating random walk (BARW) in which particles move on the lattice and evolve in discrete generations. Each particle produces a poissonian number of offspring which independently move to a uniformly chosen site within a fixed distance from their parent's position. Whenever a site is occupied by at least two particles, all the particles at that site are annihilated. This feature means that the system is not monotone and therefore the usual comparison methods are not applicable. We show that the system survives via coupling arguments and comparison with oriented percolation, making use of carefully defined density profiles which expand in time and are reminiscent of discrete travelling wave solutions. In the second part of the talk I will explain how a refinement of this technique can be employed to show complete convergence for the BARW in certain parameter regimes. The talk in based on a joint work with Matthias Birkner (JGU Mainz), Jiří Černý (University of Basel), Nina Gantert (TU Munich) and Pascal Oswald (University of Basel/JGU Mainz).

 Markus Schepers, Institut für Medizinische Biometrie, Epidemiologie und Informatik (IMBEI), UniMedizin Mainz mit dem Titel:

Cover and Hitting Times of Hyperbolic Random Graphs

Abstract: We study random walks on the giant component of Hyperbolic Random Graphs (HRGs), in the regime when the degree distribution obeys a power law with exponent in the range (2, 3). In particular, we focus on the expected times for a random walk to hit a given vertex or visit, i.e. cover, all vertices. We show that up to multiplicative constants: the cover time is n(log n)^2, the maximum hitting time is n log n, and the average hitting time is n. The first two results hold in expectation and a.a.s. and the last in expectation (with respect to the HRG).

We prove these results by determining the effective resistance either between an average vertex and the well-connected “center” of HRGs or between an appropriately chosen collection of extremal vertices. We bound the effective resistance by the energy dissipated by carefully designed network flows associated to a tiling of the hyperbolic plane on which we overlay a forest-like structure.

(joint work with Marcos Kiwi and John Sylvester)