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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
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 $F$.- James Belk and Francesco Matucci, Conjugacy and dynamics in Thompson’s groups, Geom. Dedicata 169 (2014), 239–261. MR 3175247, DOI 10.1007/s10711-013-9853-2
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