Boundary of the boundary for random walks on groups

TL;DR

This paper investigates the structure of finitely supported random walks on infinite groups, characterizing harmonic functions, dimension groups, and specific cases like the Heisenberg group to understand their boundary behaviors.

Contribution

It introduces new properties (WC and SWC) for random walks and groups, characterizes when certain dimension groups occur, and analyzes the boundary structure for specific groups like the Heisenberg group.

Findings

All abelian groups satisfy WC but not SWC.

Some abelian-by-finite groups satisfy SWC, with characterization provided.

Maximal proper space-time subcones lead to stationary simple dimension groups.

Abstract

We study fine structure related to finitely supported random walks on infinite finitely generated discrete groups, largely motivated by dimension group techniques. The unfaithful extreme harmonic functions (defined only on proper space-time cones), aka unfaithful pure traces, can be represented on systems of finite support, avoiding dead ends. This motivates properties of the random walk (WC) and of the group (SWC) which become of interest in their own right. While all abelian groups satisfy WC, the do not satisfy SWC; however some abelian by finite groups do satisfy the latter, and we characterize when this occurs. In general, we determine the maximal order ideals, aka, maximal proper space-time subcones of that generated by the group element $1$ at time zero), and show that the corresponding quotients are stationary simple dimension groups, and that all such can occur for the free group on two generators. We conclude with a case study of the discrete Heisenberg group, determining among other things, the pure traces (these are the unfaithful ones, not arising from characters).