Packing subgroups in relatively hyperbolic groups

Hruska, G Christopher¹ ; Wise, Daniel T²

¹ Department of Mathematical Sciences, University of Wisconsin–Milwaukee, PO Box 413, Milwaukee, WI 53201, USA
² Department of Mathematics and Statistics, McGill University, Montreal, Quebec H3A 2K6, Canada

Geometry & topology, Tome 13 (2009) no. 4, pp. 1945-1988.

Résumé

We introduce the bounded packing property for a subgroup of a countable discrete group $G$. This property gives a finite upper bound on the number of left cosets of the subgroup that are pairwise close in $G$. We establish basic properties of bounded packing and give many examples; for instance, every subgroup of a countable, virtually nilpotent group has bounded packing. We explain several natural connections between bounded packing and group actions on $CAT\left(0\right)$ cube complexes.

Our main result establishes the bounded packing of relatively quasiconvex subgroups of a relatively hyperbolic group, under mild hypotheses. As an application, we prove that relatively quasiconvex subgroups have finite height and width, properties that strongly restrict the way families of distinct conjugates of the subgroup can intersect. We prove that an infinite, nonparabolic relatively quasiconvex subgroup of a relatively hyperbolic group has finite index in its commensurator. We also prove a virtual malnormality theorem for separable, relatively quasiconvex subgroups, which is new even in the word hyperbolic case.

Détail

DOI : 10.2140/gt.2009.13.1945

Keywords: relative hyperbolicity, quasiconvex subgroup, width, cube complex

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