# Séminaires : Séminaire de Logique Lyon-Paris

 Equipe(s) : lm, Responsables : O. Finkel, T. Ibarlucía, A. Khélif, S. Rideau, A. Vignati Email des responsables : Salle : https://bigbluebutton.imj-prg.fr/b/sil-gwg-gge Adresse : Sophie Germain Description

 Orateur(s) Philipp Schlicht - Bristol University, Titre Oligomorphic groups are essentially countable Date 09/12/2019 Horaire 15:15 à 16:15 Diffusion Résume Model theoretic properties of a countable structure are closely connected with properties of its automorphism group. For instance, the automorphism groups of ω-categorical structures on N are precisely the oligomorphic closed subgroups of Sym(N) (a permutation group is oligomorphic if for each k there are only finitely many k-orbits). We study the complexity of topological isomorphism of oligomorphic closed subgroups of Sym(N) in the setting of Borel reducibility. Previous work of Kechris, Nies and Tent, and independently Rosendal and Zielinski, showed that this equivalence relation is below graph isomorphism. We show that it is below a Borel equivalence relation with countable equivalence classes. This is joint work with Andre Nies and Katrin Tent. Salle 2015 Adresse Sophie Germain