Uniform hyperfiniteness

Abstract:

Almost forty years ago, Connes, Feldman and Weiss proved that for measurable equivalence relations the notions of amenability and hyperfiniteness coincide. In this paper we define the uniform version of amenability and hyperfiniteness for measurable graphed equivalence relations of bounded vertex degrees and prove that these two notions coincide as well. Roughly speaking, a measured graph $\mathcal {G}$ is uniformly hyperfinite if for any ${\varepsilon }>0$ there exists $K\geq 1$ such that not only $\mathcal {G}$, but all of its subgraphs of positive measure are $({\varepsilon },K)$-hyperfinite. We also show that this condition is equivalent to weighted hyperfiniteness and a strong version of fractional hyperfiniteness, a notion recently introduced by Lovász. As a corollary, we obtain a characterization of exactness of finitely generated groups via uniform hyperfiniteness.