Powers in orbits of rational functions: cases of an arithmetic dynamical Mordell-Lang conjecture

Abstract

Let $K$ be a finitely generated field of characteristic zero. We study, for fixed $m\ge 2$, the rational functions $\varphi$ defined over $K$ that have a $K$-orbit containing infinitely many distinct $m$th powers. For $m\ge 5$ we show the only such functions are those of the form $c{x}^j(\psi (x){)}^m$ with $\psi \in K(x)$, and for $m\le 4$ we show the only additional cases are certain Lattès maps and four families of rational functions whose special properties appear not to have been studied before. With additional analysis, we show that the index set $\{n\ge 0:{\varphi }^n(a)\in \lambda ({\mathbb{P}}^1(K))\}$ is a union of finitely many arithmetic progressions, where ${\varphi }^n$ denotes the $n$th iterate of $\varphi$ and $\lambda \in K(x)$ is any map Möbius-conjugate over $K$ to $x^m$. When the index set is infinite, we give bounds on the number and moduli of the arithmetic progressions involved. These results are similar in flavor to the dynamical Mordell-Lang conjecture, and motivate a new conjecture on the intersection of an orbit with the value set of a morphism. A key ingredient in our proofs is a study of the curves $y^m={\varphi }^n(x)$. We describe all $\varphi$ for which these curves have an irreducible component of genus at most 1, and show that such $\varphi$ must have two distinct iterates that are equal in $K(x{)}^{\ast }/K(x{)}^{\ast m}$.