Almost invariance of distributions for random walks on groups

Abstract

We study the neighborhoods of a typical point $Z_n$ visited at $n$-th step of a random walk, determined by the condition that the transition probabilities stay close to $\mu^{*n}(Z_n)$. If such neighborhood contains a ball of radius $C \sqrt{n}$, we say that the random walk has almost invariant transition probabilities. We prove that simple random walks on wreath products of $\mathbb{Z}$ with finite groups have almost invariant distributions. A weaker version of almost invariance implies a necessary condition of Ozawa's criterion for the property $H_{\rm FD}$. We define and study the radius of almost invariance, we estimate this radius for random walks on iterated wreath products and show this radius can be asymptotically strictly smaller than $n/L(n)$, where $L(n)$ denotes the drift function of the random walk. We show that the radius of individual almost invariance of a simple random walk on the wreath product of $\mathbb{Z}^2$ with a finite group is asymptotically strictly larger than $n/L(n)$. Finally, we show the existence of groups such that the radius of almost invariance is smaller than a given function, but remains unbounded. We also discuss possible limiting distribution of ratios of transition probabilities on non almost invariant scales.