Rational orbits under correspondences

Abstract

Consider an algebraic function like $F(x) = \sqrt{x^3 - 1}$. If $p \in \mathbb{Q}$ is a rational number, how many iterates of $p$ under $F$ can also be rational? The dynamics of algebraic functions may be formalized in the language of correspondences on curves and their iterates. In this paper we show that if $F$ is a correspondence from $\mathbb{P}^1$ to itself defined over a finitely generated field $K$ of characteristic 0 satisfying several minor constraints, then either for each $n \geq 12$ there are only finitely many $p \in \mathbb{Q}$ for which $F^n(p)$ contains a $K$-rational point or $F$ belongs to an explicit list of known exceptional correspondences.