Counting periodic orbits of Anosov flows in free homotopy classes

TL;DR

This paper proves that in 3-manifolds supporting Anosov flows, the number of conjugacy classes of periodic orbits grows exponentially with orbit length, and characterizes flows with only finite free homotopy classes.

Contribution

It establishes exponential growth of conjugacy classes in Anosov flows and characterizes algebraic flows as the unique overed flows without infinite free homotopy classes.

Findings

Exponential growth rate equals topological entropy for transitive flows.

Shortest orbit representatives in each class relate to Bowen's measure.

Constructs examples of flows with finite and infinite free homotopy classes.

Abstract

The main result of this article is that if a $3$-manifold $M$ supports an Anosov flow, then the number of conjugacy classes in the fundamental group of $M$ grows exponentially fast with the length of the shortest orbit representative, hereby answering a question raised by Plante and Thurston in 1972. In fact we show that, when the flow is transitive, the exponential growth rate is exactly the topological entropy of the flow. We also show that taking only the shortest orbit representatives in each conjugacy classes still yields Bowen's version of the measure of maximal entropy. These results are achieved by obtaining counting results on the growth rate of the number of periodic orbits inside a free homotopy class. In the first part of the article, we also construct many examples of Anosov flows having some finite and some infinite free homotopy classes of periodic orbits, and we also give a characterization of algebraic Anosov flows as the only $\mathbb{R}$-covered Anosov flows up to orbit equivalence that do not admit at least one infinite free homotopy class of periodic orbits.