A Banach space-valued ergodic theorem for amenable groups and applications

TL;DR

This paper extends ergodic theorems to Banach space-valued functions on amenable groups, providing quantitative tiling estimates and applications to spectral theory and percolation models.

Contribution

It introduces a new abstract ergodic theorem for Banach space-valued functions on amenable groups, with quantitative tiling estimates and applications to spectral and percolation problems.

Findings

Quantitative epsilon-quasi tilings for unimodular amenable groups.

An abstract ergodic theorem for Banach space-valued functions.

Applications to density of states and cluster densities in percolation.

Abstract

In this paper we study unimodular amenable groups. The first part is devoted to results on the existence of uniform families of epsilon-quasi tilings for these groups. In this context, constructions of Ornstein and Weiss are extended by quantitative estimates for the covering properties of the corresponding decompositions. Afterwards, we apply the developed methods to obtain an abstract ergodic theorem for a class of functions mapping subsets of a countable, amenable group into some Banach space. This significantly extends and complements the previous results of Lenz, M\"uller, Schwarzenberger and Veseli\'c. Further, using the Lindenstrauss ergodic theorem, we describe a link of our results to classical ergodic theory. We conclude with two important applications: the uniform approximation of the integrated density of states on amenable Cayley graphs, as well as the almost-sure convergence of cluster densities in an amenable bond percolation model.