Topological completions of quasi-actions and discretisable spaces

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• Alex Margolis (Vanderbilt University)
• Friday 06 November 2020, 13:45-14:45
• Zoom: https://maths-cam-ac-uk.zoom.us/j/91636583222.

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A fundamental problem in geometric group theory is the study of quasi-actions. We introduce and investigate discretisable spaces: spaces for which every cobounded quasi-action can be quasi-conjugated to an isometric action on a locally finite graph. Work of Mosher-Sageev-Whyte shows that free groups are discretisable spaces, but the property holds much more generally. For instance, every non-elementary hyperbolic group is either virtually isomorphic to a cocompact lattice in rank one Lie group, or it is discretisable.

Along the way, we introduce the concept of the topological completion of a quasi-action. This is a locally compact group, well-defined up to a compact normal subgroup, reflecting the geometry of the quasi-action. We give several applications of the tools we develop. For instance we show that any finitely generated group quasi-isometric to a ‬Z‭-by-hyperbolic group is also Z-by-hyperbolic, and prove quasi-isometric rigidity for a large class of right-angled Artin groups.

This talk is part of the Geometric Group Theory (GGT) Seminar series.

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