Periods of rational maps modulo primes

Abstract

Let $K$ be a number field, let $\varphi \in K(t)$ be a rational map of degree at least 2, and let $\alpha ,\beta \in K$. We show that if $\alpha$ is not in the forward orbit of $\beta$, then there is a positive proportion of primes $\mathfrak{p}$ of $K$ such that $\alpha \mathrm{mod}\mathfrak{p}$ is not in the forward orbit of $\beta \mathrm{mod}\mathfrak{p}$. Moreover, we show that a similar result holds for several maps and several points. We also present heuristic and numerical evidence that a higher dimensional analog of this result is unlikely to be true if we replace $\alpha$ by a hypersurface, such as the ramification locus of a morphism $\varphi :{\mathbb{P}}^n\to {\mathbb{P}}^n$.

References in corpus (2)

• Questions on self maps of algebraic varieties
• The dynamical Mordell-Lang problem for etale maps