Central limit theorems for non-symmetric random walks on nilpotent covering graphs: Part II

TL;DR

This paper extends central limit theorems for non-symmetric random walks on nilpotent covering graphs by introducing a family of interpolating walks, proving semigroup and functional CLTs, and applying the results to non-harmonic realizations.

Contribution

It introduces a new family of random walks interpolating between non-symmetric and symmetric cases, and proves semigroup and functional CLTs using geometric and probabilistic techniques.

Findings

Proved a semigroup CLT for interpolating random walks.

Established a Donsker-type invariance principle on nilpotent Lie groups.

Extended CLTs to non-harmonic realizations using the corrector method.

Abstract

In the present paper, as a continuation of our preceding paper [10], we study another kind of central limit theorems (CLTs) for non-symmetric random walks on nilpotent covering graphs from a viewpoint of discrete geometric analysis developed by Kotani and Sunada. We introduce a one-parameter family of random walks which interpolates between the original non-symmetric random walk and the symmetrized one. We first prove a semigroup CLT for the family of random walks by realizing the nilpotent covering graph into a nilpotent Lie group via discrete harmonic maps. The limiting diffusion semigroup is generated by the homogenized sub-Laplacian with a constant drift of the asymptotic direction on the nilpotent Lie group, which is equipped with the Albanese metric associated with the symmetrized random walk. We next prove a functional CLT (i.e., Donsker-type invariance principle) in a Holder space over the nilpotent Lie group by combining the semigroup CLT, standard martingale techniques, and a novel pathwise argument inspired by rough path theory. Applying the corrector method, we finally extend these CLTs to the case where the realizations are not necessarily harmonic.