Abstract commensurability and quasi-isometry classification of hyperbolic surface group amalgams

TL;DR

This paper classifies hyperbolic surface group amalgams based on their commensurability and quasi-isometry, showing they are all quasi-isometric and providing geometric models for their classes.

Contribution

It characterizes the commensurability classes of hyperbolic surface group amalgams and constructs geometric models for each class, advancing understanding of their large-scale geometry.

Findings

All groups in the class are quasi-isometric.

Universal covers can be realized as isomorphic cell complexes.

Each class contains a right-angled Coxeter group.

Abstract

Let $\mathcal{X}_S$ denote the class of spaces homeomorphic to two closed orientable surfaces of genus greater than one identified to each other along an essential simple closed curve in each surface. Let $\mathcal{C}_S$ denote the set of fundamental groups of spaces in $\mathcal{X}_S$. In this paper, we characterize the abstract commensurability classes within $\mathcal{C}_S$ in terms of the ratio of the Euler characteristic of the surfaces identified and the topological type of the curves identified. We prove that all groups in $\mathcal{C}_S$ are quasi-isometric by exhibiting a bilipschitz map between the universal covers of two spaces in $\mathcal{X}_S$. In particular, we prove that the universal covers of any two such spaces may be realized as isomorphic cell complexes with finitely many isometry types of hyperbolic polygons as cells. We analyze the abstract commensurability classes within $\mathcal{C}_S$: we characterize which classes contain a maximal element within $\mathcal{C}_S$; we prove each abstract commensurability class contains a right-angled Coxeter group; and, we construct a common CAT$(0)$ cubical model geometry for each abstract commensurability class.