Diffusive estimates for random walks on stationary random graphs of polynomial growth

TL;DR

This paper proves that on stationary random graphs with polynomial growth, the random walk is at most diffusive at infinitely many times, and explores the necessity of subsequences for diffusive behavior.

Contribution

It establishes diffusive bounds for random walks on stationary random graphs of polynomial growth at infinitely many times, a result new even for subgraphs of c^d.

Findings

Random walk is at most diffusive at infinitely many times.

Stationary random graphs of polynomial growth do not admit non-constant sublinear harmonic functions.

Superdiffusive behavior can occur at an infinite subset of times, requiring subsequences for diffusive estimates.

Abstract

Let $(G,\rho)$ be a stationary random graph, and use $B^G_{\rho}(r)$ to denote the ball of radius $r$ about $\rho$ in $G$. Suppose that $(G,\rho)$ has annealed polynomial growth, in the sense that $\mathbb{E}[|B^G_{\rho}(r)|] \leq O(r^k)$ for some $k > 0$ and every $r \geq 1$. Then there is an infinite sequence of times $\{t_n\}$ at which the random walk $\{X_t\}$ on $(G,\rho)$ is at most diffusive: Almost surely (over the choice of $(G,\rho)$), there is a number $C > 0$ such that \[ \mathbb{E} \left[\mathrm{dist}_G(X_0, X_{t_n})^2 \mid X_0 = \rho, (G,\rho)\right]\leq C t_n\qquad \forall n \geq 1\,. \] This result is new even in the case when $G$ is a stationary random subgraph of $\mathbb{Z}^d$. Combined with the work of Benjamini, Duminil-Copin, Kozma, and Yadin (2015), it implies that $G$ almost surely does not admit a non-constant harmonic function of sublinear growth. To complement this, we argue that passing to a subsequence of times $\{t_n\}$ is necessary, as there are stationary random graphs of (almost sure) polynomial growth where the random walk is almost surely superdiffusive at an infinite subset of times.