Extremely amenable automorphism groups of countable structures (2411.02760v1)

Abstract: In this paper we address the question: How many pairwise non-isomorphic extremely amenable groups are there which are seprable 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 classicfication problem from the point of view of descriptive set theory by showing that the class of all extremely amenable closed subgroups of $S_\infty$ is Borel and by obtaining some lower bounds for their isomorphism relation in the Borel reducibility hierarchy.