Duke Mathematical Journal

On the cubical geometry of Higman’s group

Alexandre Martin

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We investigate the cocompact action of Higman’s group on a CAT(0) square complex associated to its standard presentation. We show that this action is in a sense intrinsic, which allows for the use of geometric techniques to study the endomorphisms of the group, and we show striking similarities with mapping class groups of hyperbolic surfaces, outer automorphism groups of free groups, and linear groups over the integers. We compute explicitly the automorphism group and outer automorphism group of Higman’s group and show that the group is both Hopfian and co-Hopfian. We actually prove a stronger rigidity result about the endomorphisms of Higman’s group: every nontrivial morphism from the group to itself is an automorphism. We also study the geometry of the action and prove a surprising result: although the CAT(0) square complex acted upon contains uncountably many flats, the Higman group does not contain subgroups isomorphic to Z2. Finally, we show that this action possesses features reminiscent of negative curvature, which we use to prove a refined version of the Tits alternative for Higman’s group.

Article information

Duke Math. J., Volume 166, Number 4 (2017), 707-738.

Received: 19 August 2015
Revised: 26 May 2016
First available in Project Euclid: 3 January 2017

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Mathematical Reviews number (MathSciNet)

Zentralblatt MATH identifier

Primary: 20F65: Geometric group theory [See also 05C25, 20E08, 57Mxx]
Secondary: 20F28: Automorphism groups of groups [See also 20E36]

Higman group $\operatorname{CAT}(0)$ cube complexes automorphism group Hopfian groups co-Hopfian groups Tits alternative


Martin, Alexandre. On the cubical geometry of Higman’s group. Duke Math. J. 166 (2017), no. 4, 707--738. doi:10.1215/00127094-3715913. https://projecteuclid.org/euclid.dmj/1483412430

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