Triple collisions on a comb graph

Abstract

In this article, we consider the number of collisions of three independent simple random walks on a subgraph of the two-dimensional square lattice obtained by removing all horizontal edges with vertical coordinate not equal to 0 and then, for $n\in \mathbb{Z}$, restricting the vertical segment of the graph located at horizontal coordinate $n$ to the interval $\{0,1,\dots,\log^{\alpha}(|n|\vee 1)\}$. Specifically, we show the following phase transition: when $\alpha\leq 1$, the three random walks collide infinitely many times almost-surely, whereas when $\alpha>1$, they collide only finitely many times almost-surely. This is a variation of a result of Barlow, Peres and Sousi, who showed a similar phase transition for two random walks when the vertical segments are truncated at height $|n|^{\alpha}$.