Rational Preperiodic Points of Quadratic Rational Maps over $\mathbb{Q}$ with Nonabelian Automorphism Groups

Abstract

Let $f:{\mathbb{P}}^1\to {\mathbb{P}}^1$ be a quadratic rational map defined over the rational field $\mathbb{Q}$ with nonabelian automorphism group. We prove that no such map has a $\mathbb{Q}$-rational periodic point with exact period $N\ge 4$. We also give an explicit parametrization of such maps that have $\mathbb{Q}$-rational periodic points of period $1$, $2$, and $3$. In addition, we show that the number of $\mathbb{Q}$-rational preperiodic points of such a map $f$ cannot exceed $6$. As a result, we completely classify all portraits of $\mathbb{Q}$-rational preperiodic points for quadratic rational maps defined over $\mathbb{Q}$ with nonabelian automorphism showing that there are exactly $5$ such portraits.

arXiv comment16 pages. Includes 6 directed graphs of preperiodic orbits