Piecewise Visual, Linearly Connected Metrics on Boundaries of Relatively Hyperbolic Groups

TL;DR

This paper develops piecewise visual, linearly connected metrics on the boundaries of relatively hyperbolic groups with cut points, extending the understanding of boundary metrics beyond the no cut point case.

Contribution

It introduces a method to construct piecewise visual, linearly connected metrics on boundaries of relatively hyperbolic groups with cut points, generalizing previous results.

Findings

Constructed piecewise visual metrics on boundaries with cut points.

Established conditions for linearly connected metrics based on group decompositions.

Extended the class of boundaries where visual metrics can be applied.

Abstract

Suppose a finitely generated group $G$ is hyperbolic relative to $\mathcal P$ a set of proper finitely generated subgroups of $G$. Established results in the literature imply that a "visual" metric on $\partial (G,\mathcal P)$ is "linearly connected" if and only if the boundary $\partial (G,\mathcal P)$ has no cut point. Our goal is to produce linearly connected metrics on $\partial (G,\mathcal P)$ that are "piecewise" visual when $\partial (G,\mathcal P)$ contains cut points. %Visual metrics for $\partial (G,\mathcal P)$ are tightly linked to inner products of geodesic rays in "cusped" spaces for $(G,\mathcal P)$. The identity vertex $\ast$ is usually our base point in these cusped spaces and visual metrics depend on this base point. %We say the visual metric $d_p$ on $\partial(G,\mathcal P)$, with base point $p$, is {\it $G$-equivariant} if for points $x_1,x_2\in \partial(G,\mathcal P)$, we have $d_p(x_1,x_2)=d_{gp}(gx_1,gx_2)$ for all $g\in G$. Our main theorem is about graph of groups decompositions of relatively hyperbolic groups $(G,\mathcal P)$, and piecewise visual metrics on their boundaries. We assume that each vertex group of our decomposition has a boundary with linearly connected visual metric or the vertex group is in $\mathcal P$. If a vertex group is not in $\mathcal P$, then it is hyperbolic relative to its adjacent edge groups. Our linearly connected metric on $\partial (G,\mathcal P)$ agrees with the visual metric on limit sets of vertex groups and is in this sense piecewise visual.