Bound for preperiodic points for maps with good reduction

Abstract

Let $K$ be a number field and let $\phi$ in $K(z)$ be a rational function of degree $d\geq 2$. Let $S$ be the places of bad reduction for $\phi$ (including the archimedan places). Let $Per(\phi,K)$, $PrePer(\phi, K)$, and $Tail(\phi,K)$ be the set of $K$-rational periodic, preperiodic, and purely preperiodic points of $\phi$, respectively. The present paper presents two main results. The first result gives a bound for $|PrePer(\phi,K)|$ in terms of the number of places of bad reduction $|S|$ and the degree $d$ of the rational function $\phi$. This bound significantly improves a previous bound given by J. Canci and L. Paladino 2014. For the second result, assuming that $|Per(\phi,K)| \geq 4$ (resp. $|Tail(\phi,K)| \geq 3$), we prove bounds for $|Tail(\phi,K)|$ (resp. $|Per(\phi,K)|$) that depend only on the number of places of bad reduction $|S|$ (and not on the degree $d$). We show that the hypotheses of this result are sharp, giving counterexamples to any possible result of this form when $|Per(\phi,K)| < 4$ (resp. $|Tail(\phi,K)| < 3$).