The boundary of the range of a random walk and the Følner property

Abstract

The range process $R_n$ of a random walk is the collection of sites visited by the random walk up to time n. In this paper we deal with the question of whether the range process of a random walk or the range process of a cocycle over an ergodic transformation is almost surely a Følner sequence and show the following results: (a) The size of the inner boundary $|\partial R_n|$ of the range of recurrent aperiodic random walks on ${\mathbb{Z}}^2$ with finite variance and aperiodic random walks in $\mathbb{Z}$ in the standard domain of attraction of the Cauchy distribution, divided by $n∕{log}^2(n)$, converges to a constant almost surely. (b) We establish a formula for the Følner asymptotic of transient cocycles over an ergodic probability preserving transformation and use it to show that for admissible transient random walks on finitely generated groups, the range is never a Følner sequence unless the walk is a skip-free random walk on $\mathbb{Z}$. (c) For strongly aperiodic random walks in the domain of attraction of symmetric α-stable distributions with $1<\mathit{\alpha }\le 2$, we prove a sharp polynomial upper bound for the decay at infinity of $|\partial R_n|∕|R_n|$. This last result shows that the range process of these random walks is almost surely a Følner sequence.