Cusped spaces and quasi-isometries of relatively hyperbolic groups

TL;DR

This paper demonstrates that relatively hyperbolic groups have quasi-isometric cusped spaces under certain conditions, establishing a canonical boundary structure and characterizing lattices in negatively curved symmetric spaces.

Contribution

It proves the quasi-isometry invariance of cusped spaces for relatively hyperbolic groups with constant horospherical distortion and characterizes lattices via these spaces.

Findings

Any two cusped spaces with constant horospherical distortion are quasi-isometric.

The Bowditch boundary admits a canonical quasisymmetric structure.

A group is a lattice in a negatively curved symmetric space if its cusped space is quasi-isometric to that space.

Abstract

A group $\Gamma$ with a family of subgroups $\mathbb{P}$ is relatively hyperbolic if $\Gamma$ admits a cusp-uniform action on a proper $\delta$--hyperbolic space. We show that any two such spaces for a given group pair are quasi-isometric, provided the spaces have "constant horospherical distortion," a condition satisfied by Groves--Manning's cusped Cayley graph and by all negatively curved symmetric spaces. Consequently the Bowditch boundary admits a canonical quasisymmetric structure, which coincides with the "naturally occurring" quasisymmetric structure of the symmetric space when considering lattices in rank one symmetric spaces. We show that a group $\Gamma$ is a lattice in a negatively curved symmetric space $X$ if and only if a cusped space for $\Gamma$ is quasi-isometric to the symmetric space. We also prove an ideal triangle characterization of the $\delta$--hyperbolic spaces with uniformly perfect boundary due to Meyer and Bourdon--Kleiner. An appendix concerns the equivalence of several definitions of conical limit point found in the literature.