The average number of integral points in orbits

Abstract

Over a number field $K$, a celebrated result of Silverman states that if $φ(z)\in K(z)$ is a rational function whose second iterate is not a polynomial, the set of $S$-integral points in the orbit $\text{Orb}_φ(P)=\{φ^n(P)\}_{n\geq0}$ is finite for all $P\in \mathbb{P}^1(K)$. In this paper, we show that if we vary $φ$ and $P$ in a suitable family, the number of $S$-integral points in $\text{Orb}_φ(P)$ is absolutely bounded. In particular, if we fix $φ$ and vary the basepoint $P\in \mathbb{P}^1(K)$, then we show that $\#(\text{Orb}_φ(P)\cap\mathcal{O}_{K,S})$ is zero on average. Finally, we prove a zero-average result in general, assuming a standard height uniformity conjecture in arithmetic geometry.