English

Intersection-saturated groups without free subgroups

Group Theory 2023-12-19 v2

Abstract

A group GG is said to be intersection-saturated if for every strictly positive integer nn and every map c ⁣:P({1,,n}){0,1}c\colon \mathcal{P}(\{1,\dots, n\})\setminus \emptyset \rightarrow \{0,1\}, one can find subgroups H1,,HnGH_1,\dots, H_n\leq G such that for every non-empty subset I{1,,n}I\subseteq \{1,\dots, n\}, the intersection iIHi\bigcap_{i\in I}H_i is finitely generated if and only if c(I)=0c(I)=0. We obtain a new criterion for a group to be intersection-saturated based on the existence of arbitrarily high direct powers of a subgroup admitting an automorphism with a non-finitely generated set of fixed points. We use this criterion to find new examples of intersection-saturated groups, including Thompson's groups and the Grigorchuk group. In particular, this proves the existence of finitely presented intersection-saturated groups without non-abelian free subgroups, thus answering a question of Delgado, Roy and Ventura.

Keywords

Cite

@article{arxiv.2312.09954,
  title  = {Intersection-saturated groups without free subgroups},
  author = {Dominik Francoeur},
  journal= {arXiv preprint arXiv:2312.09954},
  year   = {2023}
}

Comments

7 pages

R2 v1 2026-06-28T13:52:39.756Z