Scarcity of finite orbits for rational functions over a number field

Abstract

Let $\phi$ be a an endomorphism of degree $d\geq{2}$ of the projective line, defined over a number field $K$. Let $S$ be a finite set of places of $K$, including the archimedean places, such that $\phi$ has good reduction outside of $S$. The article presents two main results: the first result is a bound on the number of $K$-rational preperiodic points of $\phi$ in terms of the cardinality of the set $S$ and the degree $d$ of the endomorphism $\phi$. This bound is quadratic in terms of $d$ which is a significant improvement to all previous bounds on the number of preperiodic points in terms of the degree $d$. For the second result, if we assume that there is a $K$-rational periodic point of period at least two, then there exists a bound on the number of $K$-rational preperiodic points of $\phi$ that is linear in terms of the degree $d$.