Amenability and Unique Ergodicity of Automorphism Groups of Fra\"iss\'e Structures

Abstract

In this paper we provide a necessary and sufficient condition for the amenability of the automorphism group of Fra\"iss\'e structures and apply it to prove the non-amenability of the automorphism groups of the directed graph $\mathbf{S}(3)$ and the Boron tree structure $\mathbf{T}$. Also, we provide a negative answer to the Unique Ergodicity-Generic Point problem of Angel-Kechris-Lyons [AKL]. By considering $\mathrm{GL}(\mathbf{V}_\infty)$, where $\mathbf{V}_\infty$ is the countably infinite dimensional vector space over a finite field $F_q$, we show that the unique invariant measure on the universal minimal flow of $\mathrm{GL}(\mathbf{V}_\infty)$ is not supported on the generic orbit.