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Free actions of free groups on countable structures and property (T)

Volume 232 / 2016

David M. Evans, Todor Tsankov Fundamenta Mathematicae 232 (2016), 49-63 MSC: Primary 22A25, 20B27; Secondary 03C15. DOI: 10.4064/fm232-1-4


We show that if $G$ is a non-archimedean, Roelcke precompact Polish group, then $G$ has Kazhdan's property (T). Moreover, if $G$ has a smallest open subgroup of finite index, then $G$ has a finite Kazhdan set. Examples of such $G$ include automorphism groups of countable $\omega $-categorical structures, that is, the closed, oligomorphic permutation groups on a countable set. The proof uses work of the second author on the unitary representations of such groups, together with a separation result for infinite permutation groups. The latter allows the construction of a non-abelian free subgroup of $G$ acting freely in all infinite transitive permutation representations of $G$.


  • David M. EvansDepartment of Mathematics
    Imperial College London
    London SW7 2AZ, UK
  • Todor TsankovUniversité Paris 7
    UFR de Mathématiques, Case 7012
    75205 Paris Cedex 13, France

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