Automorphism Groups of Generic Structures: Extreme Amenability and Amenability

TL;DR

This paper explores the relationship between extreme amenability and amenability of automorphism groups of certain generic structures, establishing non-amenability results for specific classes derived from pre-dimension functions.

Contribution

It establishes new connections between amenability properties and Ramsey-type properties of Fra"iss"e-Hrushovski structures, and proves non-amenability of automorphism groups in key cases.

Findings

Automorphism groups of ordered Hrushovski generic graphs are not extremely amenable.

Automorphism groups of certain Fra"iss"e-Hrushovski structures are not amenable.

The results extend to structures obtained from pre-dimension functions with rational coefficients.

Abstract

We investigate correspondences between extreme amenability and amenability of automorphism groups of Fra\"iss\'e-Hrushovski generic structures that are obtained from smooth classes, and their Ramsey type properties of their smooth classes, similar to Kechris, Pestov and Todorcevic, and Tatch Moore. In particular, we focus on some Fra\"iss\'e-Hrushovski generic structures that are obtained from pre-dimension functions. Using these correspondences, we prove that automorphism groups of ordered Hrushovski generic graphs are not extremely amenable in both cases of collapsed and uncollapsed. Moreover, we prove that automorphism groups of Fra\"iss\'e-Hrushovski generic structures that are obtained from pre-dimension functions with rational coefficients are not amenable.