Amenability, Optimal Transport and Abstract Ergodic Theorems

Abstract

Using tools from the theory of optimal transport, we establish several results concerning isometric actions of amenable topological groups with potentially unbounded orbits. Specifically, suppose $d$ is a compatible left-invariant metric on an amenable topological group $G$ with no non-trivial homomorphisms to $\mathbb R$. Then, for every finite subset $E\subseteq G$ and $\epsilon>0$, there is a finitely supported probability measure $\beta$ on $G$ such that $$ \max_{g,h\in E}\, {\sf W}(\beta g, \beta h)<\epsilon, $$ where ${\sf W}$ denotes the Wasserstein distance between probability measures on the metric space $(G,d)$. When $d$ is the word metric on a finitely generated group $G$, this strengthens a well known theorem of Reiter and, when $d$ is bounded, recovers a result of Schneider and Thom. Furthermore, when $G$ is locally compact, $\beta$ may be replaced by an appropriate probability density $f\in L^1(G)$. Also, when $G\curvearrowright X$ is a continuous isometric action on a metric space, the space of Lipschitz functions on the quotient $X/\!\!/G$ is isometrically isomorphic to a $1$-complemented subspace of the Lipschitz functions on $X$. And, when additionally $G$ is skew-amenable, there is a $G$-invariant contraction $$ \mathfrak {Lip}\, X \overset S\longrightarrow\mathfrak{Lip}(X/\!\!/G) $$ so that $(S\phi\big)\big(\overline{Gx}\big)=\phi(x)$ whenever $\phi$ is constant on every orbit of $G\curvearrowright X$. This latter extends results of Cuth and Doucha from the setting of locally compact or balanced groups.