Periodic points of rational functions over finite fields

Abstract

For $q$ a prime power and $ϕ$ a rational function with coefficients in $\mathbb{F}_q$, let $p(q,ϕ)$ be the proportion of $\mathbb{P}^1(\mathbb{F}_q)$ that is periodic with respect to $ϕ$. And if $d$ is a positive integer, let $Q_d$ be the set of prime powers coprime to $d!$ and let $\mathcal{P}(d,q)$ be the expected value of $p(q,ϕ)$ as $ϕ$ ranges over rational functions with coefficients in $\mathbb{F}_q$ of degree $d$. We prove that if $d$ is a positive integer no less than $2$, then $\mathcal{P}(d,q)$ tends to 0 as $q$ increases in $Q_d$. This theorem generalizes our previous work, which held only for quadratic polynomials, and only in fixed characteristic. To deduce this result, we prove a uniformity theorem on specializations of dynamical systems of rational functions with coefficients in certain finitely-generated algebras over residually finite Dedekind domains. This specialization theorem generalizes our previous work, which held only for algebras of dimension one.