Extremely amenable automorphism groups of countable structures
Logic
2026-03-23 v2 General Topology
Abstract
In this paper we address the question: How many pairwise non-isomorphic extremely amenable groups are there which are separable metrizable or even Polish? We show that there are continuum many such groups. In fact we construct continuum many pairwise non-isomorphic extremely amenable groups as automorphism groups of countable structures. We also consider this classification problem from the point of view of descriptive set theory by showing that the class of all extremely amenable closed subgroups of is Borel and their isomorphism relation is more complex than any isomorphism relation of countable structures in the Borel reducibility hierarchy.
Cite
@article{arxiv.2411.02760,
title = {Extremely amenable automorphism groups of countable structures},
author = {Mahmood Etedadialiabadi and Su Gao and Feng Li and Ruiwen Li},
journal= {arXiv preprint arXiv:2411.02760},
year = {2026}
}