Groups, Geometry, and Dynamics

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Volume 8, Issue 3, 2014, pp. 837–861
DOI: 10.4171/GGD/250

Published online: 2014-10-02

On transitivity and (non)amenability of Aut $F_n$ actions on group presentations

Aglaia Myropolska[1] and Tatiana Nagnibeda[2]

(1) Université de Genève, Switzerland
(2) Université de Genève, Switzerland

For a finitely generated group $G$ the Nielsen graph $N_n(G)$, $n\geq \operatorname{rank}(G)$, describes the action of the group $\operatorname{Aut}F_n$ of automorphisms of the free group $F_n$ on generating $n$-tuples of G by elementary Nielsen moves. The question of (non)amenability of Nielsen graphs is of particular interest in relation with the open question about Property $(T)$ for $\operatorname{Aut}F_n$, $n\geq 4$. We prove nonamenability of Nielsen graphs $N_n(G)$ for all $n\ge \max\{2,\operatorname{rank}(G)\}$ when $G$ is indicable, and for $n$ big enough when $G$ is elementary amenable. We give an explicit description of $N_d(G)$ for relatively free (in some variety) groups of rank $d$ and discuss their connectedness and nonamenability. Examples considered include free polynilpotent groups and free Burnside groups.

Keywords: Automorphisms group of a free group, generating set, transitive action, amenable graph

Myropolska Aglaia, Nagnibeda Tatiana: On transitivity and (non)amenability of Aut $F_n$ actions on group presentations. Groups Geom. Dyn. 8 (2014), 837-861. doi: 10.4171/GGD/250