Mapping classes of Real Rational Surface Automorphisms

TL;DR

This paper studies the mapping classes of certain real rational surface automorphisms, revealing their pseudo-Anosov nature and providing a new geometric realization of Lehmer's number as a stretch factor.

Contribution

It determines the mapping classes of specific diffeomorphisms on real rational surfaces, showing they are pseudo-Anosov and not from Penner's construction, and realizes Lehmer's number as a stretch factor.

Findings

The diffeomorphisms are reducible with unique invariant curves.

Their restrictions are pseudo-Anosov and not from Penner's construction.

Lehmer's number is realized as the stretch factor for n=8.

Abstract

Let $\{F_n, n\ge 8\}$ be a family of diffeomorphisms on real rational surfaces that are birationally equivalent to birational maps on $\mathbf{P}^2(\mathbb{R})$. In this article, we investigate the mapping classes of the diffeomorphisms $F_n, n\ge 8$. These diffeomorphisms are reducible with unique invariant irreducible curves, and we determine the mapping classes of their restrictions, $\hat F_n, n \ge 8$, on the cut surfaces, showing that they are pseudo-Anosov and do not arise from Penner's construction. For $n=8$, Lehmer's number is realized as the stretch factor of $\hat F_8$, a pseudo-Anosov map on a once-punctured genus $5$ orientable surface. The diffeomorphism $\hat F_8$ is a new geometric realization of a Lehmer's number.