Periods of rational maps modulo primes
  Let $K$ be a number field, let $\phi \in K(t)$ 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 $\alpha \mod {\mathfrak p}$ is not in the forward orbit of $\beta \mod {\mathfrak p}$.   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 $\phi : {\mathbb P}^{n} \to {\mathbb P}^{n}$. 
Robert L. Benedetto, Dragos Ghioca, Benjamin Hutz, P\"ar Kurlberg,   Thomas Scanlon, and Thomas J. Tucker
math.AG math.NT