Statistical hyperbolicity in groups

TL;DR

This paper introduces the 'sprawl' statistic to measure how groups deviate from hyperbolicity, relating it to curvature, and provides algorithms and results for various group types, including abelian and hyperbolic groups.

Contribution

It defines the sprawl metric, relates it to curvature, and offers an algorithm to compute it, advancing understanding of hyperbolicity in different groups.

Findings

Hyperbolic groups are statistically hyperbolic.

Algorithm computes sprawl exactly for any generating set.

Connections to convex geometry and Mahler conjecture.

Abstract

In this paper, we introduce a geometric statistic called the "sprawl" of a group with respect to a generating set, based on the average distance in the word metric between pairs of words of equal length. The sprawl quantifies a certain obstruction to hyperbolicity. Group presentations with maximum sprawl (i.e., without this obstruction) are called statistically hyperbolic. We first relate sprawl to curvature and show that nonelementary hyperbolic groups are statistically hyperbolic, then give some results for products, for Diestel-Leader graphs and lamplighter groups. In free abelian groups, the word metrics asymptotically approach norms induced by convex polytopes, causing the study of sprawl to reduce to a problem in convex geometry. We present an algorithm that computes sprawl exactly for any generating set, thus quantifying the failure of various presentations of Z^d to be hyperbolic. This leads to a conjecture about the extreme values, with a connection to the classic Mahler conjecture.