Patterson-Sullivan measures and growth of relatively hyperbolic groups

Wenyuan Yang

Published 2013-08-28, updated 2015-07-19Version 4

We prove that for a relatively hyperbolic group G there is a sequence of relatively hyperbolic proper quotients such that their growth rates converge to the growth rate of G. Under natural assumptions, the same conclusion holds for the critical exponent of a cusp-uniform action of G on a hyperbolic metric space. As a corollary, we obtain that the critical exponent of a torsion-free geometrically finite Kleinian group can be arbitrarily approximated by those of quotient groups. This resolves a question of Dal'bo-Peign\'e-Picaud-Sambusetti. Our approach is based on the study of Patterson-Sullivan measures on Bowditch boundary of a relatively hyperbolic group. The uniqueness and ergodicity of Patterson-Sullivan measures are proved when the group is divergent. We prove a variant of the Sullivan Shadow Lemma, called Partial Shadow Lemma. This tool allows us to prove several results on growth functions of balls and cones. One central result is the existence of a sequence of geodesic trees with growth rates sufficiently close to the growth rate of G, and transition points uniformly spaced in trees. By taking small cancellation over hyperbolic elements of high powers, we can embed these trees into properly constructed quotients of G. This proves the results in the previous paragraph.