Real entropy rigidity under quasi-conformal deformations

TL;DR

This paper investigates the behavior of real entropy in the space of real rational maps, proving local rigidity under quasi-conformal deformations and classifying maps with maximal real entropy.

Contribution

It establishes that the real entropy function is locally constant on certain conjugacy classes and provides a classification of maps with maximal real entropy.

Findings

Real entropy is locally constant under quasi-conformal conjugation.

Classification of maps with maximal real entropy.

Analysis of hyperbolic and Lattès maps with real coefficients.

Abstract

We set up a real entropy function $h_\Bbb{R}$ on the space $\mathcal{M}'_d$ of M\"obius conjugacy classes of real rational maps of degree $d$ by assigning to each class the real entropy of a representative $f\in\Bbb{R}(z)$; namely, the topological entropy of its restriction $f\restriction_{\hat{\Bbb{R}}}$ to the real circle. We prove a rigidity result stating that $h_\Bbb{R}$ is locally constant on the subspace determined by real maps quasi-conformally conjugate to $f$. As examples of this result, we analyze real analytic stable families of hyperbolic and flexible Latt\`es maps with real coefficients along with numerous families of degree $d$ real maps of real entropy $\log(d)$. The latter discussion moreover entails a complete classification of maps of maximal real entropy.