Real Rational Surface Automorphisms : Positivity and Linearity

TL;DR

This paper investigates the dynamics of certain rational surface automorphisms, revealing exponential growth and positive entropy, and connects these to pseudo-Anosov homeomorphisms via combinatorial and algebraic methods.

Contribution

It introduces a novel positivity framework and a combinatorial induction principle to analyze real rational surface automorphisms and their complex dynamics.

Findings

Exponential growth rate $>1$ for the induced action on the fundamental group.

The real map exhibits positive topological entropy.

The induced automorphism is realized by a pseudo-Anosov homeomorphism.

Abstract

We study the real dynamics of a family of rational surface automorphisms obtained from quadratic birational maps of $\pcc$ that preserve a cuspidal cubic and whose critical orbits have lengths $(1,m,n)$ with $1+m+n\ge 10$. Passing to the real locus and cutting along the invariant cubic, we obtain a diffeomorphism of an orientable surface whose fundamental group is free. Our key device is a finitely generated invariant, positive semigroup $S_{m,n}$ in the fundamental group on which an iterate of induced action acts by concatenation without cancellation. This positivity yields a nonnegative primitive transition matrix, so Perron-Frobenius theory supplies an explicit exponential growth rate $\lambda>1$ for the induced action on the fundamental group. Consequently, the real map has positive topological entropy. We package the combinatorics of the generators in a ``Core-Tail Induction Principle," which allows us to treat simultaneously seven orbit-data families with only finite base checks. Finally, using Bestvina-Handel and the Dehn-Nielsen-Baer correspondence, we show that the induced outer automorphism with $m+n$ odd is realized by a pseudo-Anosov homeomorphism of the cut surface.