Counting double cosets with application to generic 3-manifolds

TL;DR

This paper investigates the growth of double cosets in groups with contracting elements, establishing new growth comparisons and confirming a conjecture about the genericity of hyperbolic 3-manifolds within certain 3-manifold classes.

Contribution

It generalizes existing results on hyperbolic groups to broader classes and proves a conjecture regarding the exponential genericity of hyperbolic 3-manifolds.

Findings

Double coset growth is comparable to orbital growth in these groups.

Confirmed Maher's conjecture on hyperbolic 3-manifolds being exponentially generic.

Extended growth results to subgroups with proper limit sets.

Abstract

We study the growth of double cosets in the class of groups with contracting elements, including relatively hyperbolic groups, CAT(0) groups and mapping class groups among others. Generalizing a recent work of Gitik and Rips about hyperbolic groups, we prove that the double coset growth of two Morse subgroups of infinite index is comparable with the orbital growth function. The same result is further obtained for a more general class of subgroups whose limit sets are proper subsets in the entire limit set of the ambient group. As an application, we confirm a conjecture of Maher that hyperbolic 3-manifolds are exponentially generic in the set of 3-manifolds built from Heegaard splitting using complexity in Teichm\"{u}ller metric.