English

From isolated subgroups to generic permutation representations

Group Theory 2017-05-17 v1

Abstract

Let GG be a countable group, Sub(G)\operatorname{Sub}(G) the (compact, metric) space of all subgroups of GG with the Chabauty topology and Is(G)Sub(G)\operatorname{Is}(G) \subset \operatorname{Sub}(G) the collection of isolated points. We denote by X!X! the (Polish) group of all permutations of a countable set XX. Then the following properties are equivalent: (i) Is(G)\operatorname{Is}(G) is dense in Sub(G)\operatorname{Sub}(G), (ii) GG admits a "generic permutation representation". Namely there exists some τHom(G,X!)\tau^* \in \operatorname{Hom}(G,X!) such that the collection of permutation representations {ϕHom(G,X!)  ϕis permutation isomorphic toτ}\{\phi \in \operatorname{Hom}(G,X!) \ | \ \phi {\text{is permutation isomorphic to}} \tau^*\} is co-meager in Hom(G,X!)\operatorname{Hom}(G,X!). We call groups satisfying these properties solitary. Examples of solitary groups include finitely generated LERF groups and groups with countably many subgroups.

Keywords

Cite

@article{arxiv.1601.07538,
  title  = {From isolated subgroups to generic permutation representations},
  author = {Yair Glasner and Daniel Kitroser and Julien Melleray},
  journal= {arXiv preprint arXiv:1601.07538},
  year   = {2017}
}

Comments

21 pages

R2 v1 2026-06-22T12:38:06.263Z