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The Generation Problem in Thompson Group $F$

About this Title

Gili Golan Polak

Publication: Memoirs of the American Mathematical Society
Publication Year: 2023; Volume 292, Number 1451
ISBNs: 978-1-4704-6723-4 (print); 978-1-4704-7696-0 (online)
DOI: https://doi.org/10.1090/memo/1451
Published electronically: November 17, 2023
Keywords: Thompson’s group $F$, decision problems, diagram groups, directed $2$-complexes, the Stallings $2$-core, closed subgroups, maximal subgroups, homeomorphisms of the interval

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Table of Contents

Chapters

  • 1. Introduction
  • 2. Preliminaries on $F$
  • 3. The Stallings $2$-core of subgroups of diagram groups
  • 4. Paths on the core of a subgroup $H\le F$
  • 5. The closure of a subgroup $H\le F$
  • 6. Transitivity of the action of $H$ on the set $\mathcal D$
  • 7. The generation problem in $F$
  • 8. The Tuples algorithm
  • 9. $F$ is a cyclic extension of a subgroup $K$ which has a maximal elementary amenable subgroup
  • 10. Computations related to $\mathcal {L}(H)$
  • 11. Solvable subgroups of Thompson’s group $F$
  • 12. Open problems

Abstract

We show that the generation problem in Thompson’s group $F$ is decidable, i.e., there is an algorithm which decides if a finite set of elements of $F$ generates the whole $F$. The algorithm makes use of the Stallings $2$-core of subgroups of $F$, which can be defined in an analogous way to the Stallings core of subgroups of a finitely generated free group. Further study of the Stallings $2$-core of subgroups of $F$ provides a solution to another algorithmic problem in $F$. Namely, given a finitely generated subgroup $H$ of $F$, it is decidable if $H$ acts transitively on the set of finite dyadic fractions $\mathcal D$. Other applications of the study include the construction of new maximal subgroups of $F$ of infinite index, among which, a maximal subgroup of infinite index which acts transitively on the set $\mathcal D$ and the construction of an elementary amenable subgroup of $F$ which is maximal in a normal subgroup of $F$.

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