Cycles for rational maps with good reduction outside a prescribed set

Abstract

Let $K$ be a number field and $S$ a fixed finite set of places of $K$ containing all the archimedean ones. Let $R_S$ be the ring of $S$-integers of $K$. In the present paper we study the cycles for rational maps of ${\mathbb{P}}_1(K)$ of degree $\ge 2$ with good reduction outside $S$. We say that two ordered $n$-tuples $(P_0,P_1,...,P_{n-1})$ and $(Q_0,Q_1,...,Q_{n-1})$ of points of ${\mathbb{P}}_1(K)$ are equivalent if there exists an automorphism $A\in {\mathrm{PGL}}_2(R_S)$ such that $P_i=A(Q_i)$ for every index $i\in \{0,1,...,n-1\}$. We prove that if we fix two points $P_0,P_1\in {\mathbb{P}}_1(K)$, then the number of inequivalent cycles for rational maps of degree $\ge 2$ with good reduction outside $S$ which admit $P_0,P_1$ as consecutive points is finite and depends only on $S$. We also prove that this result is in a sense best possible.

30 pages, changed content