Geometric entropy of geodesic currents on free groups

TL;DR

This paper introduces and analyzes the concept of geometric entropy for geodesic currents on free groups, revealing how it relates to length functions, volume entropy, and extremal distortions, and establishing its maximum at specific points in outer space.

Contribution

It defines geometric entropy for geodesic currents on free groups and provides explicit formulas and properties, connecting it with volume entropy and extremal distortions in outer space.

Findings

Explicit formula for geometric entropy involving volume entropy and extremal distortion.

Maximum of geometric entropy achieved at the same point as the current's defining point.

Inequality relating geometric entropy and volume entropy, with equality for Patterson-Sullivan currents.

Abstract

A \emph{geodesic current} on a free group $F$ is an $F$-invariant measure on the set $\partial^2 F$ of pairs of distinct points of $\partial F$. The space of geodesic currents on $F$ is a natural companion of Culler-Vogtmann's Outer space $cv(F)$ and studying them together yields new information about both spaces as well as about the group $Out(F)$. The main aim of this paper is to introduce and study the notion of {\it geometric entropy} $h_T(\mu)$ of a geodesic current $\mu$ with respect to a point $T$ of $cv(F)$, which can be viewed as a length function on $F$. The geometric entropy is defined as the slowest rate of exponential decay of $\mu$-measures of bi-infinite cylinders in $F$, as the $T$-length of the word defining such a cylinder goes to infinity. We obtain an explicit formula for $h_{T'}(\mu_T)$, where $T,T'$ are arbitrary points in $cv(F)$ and where $\mu_T$ denotes a Patterson-Sullivan current corresponding to $T$. It involves the volume entropy $h(T)$ and the extremal distortion of distances in $T$ with respect to distances in $T'$. It follows that, given $T$ in the projectivized outer space $CV(F)$, $h_{T'}(\mu_T)$ as function of $T'\in CV(F)$ achieves a strict global maximum at $T'=T$. We also show that for any $T\in cv(F)$ and any geodesic current $\mu$ on $F$, $h_T(\mu)\le h(T)$, where the equality is realized when $\mu=\mu_T$. For points $T\in cv(F)$ with simplicial metric (where all edges have length one), we relate the geometric entropy of a current and the measure-theoretic entropy.