Integral points and orbits of endomorphisms on the projective plane

Abstract

We analyze when integral points on the complement of a finite union of curves in $\mathbb{P}^2$ are potentially dense. We divide the analysis of these affine surfaces based on their logarithmic Kodaira dimension $\bar{\kappa}$. When $\bar{\kappa} = -\infty$, we completely characterize the potential density of integral points in terms of the number of irreducible components on the surface at infinity and the number of multiple members in a pencil naturally associated to the surface. When integral points are not potentially dense, we show that they lie on finitely many effectively computable curves. When $\bar{\kappa} = 0$, we prove that integral points are always potentially dense. The bulk of our analysis concerns the subtle case of $\bar{\kappa}=1$. We determine the potential density of integral points in a number of cases and develop tools for studying integral points on surfaces fibered over a curve. Finally, nondensity of integral points in the case $\bar{\kappa}=2$ is predicted by the Lang-Vojta conjecture, to which we have nothing new to add. In a related direction, we study integral points in orbits under endomorphisms of $\mathbb{P}^2$. Assuming the Lang--Vojta conjecture, we prove that an orbit under an endomorphism $\phi$ of $\mathbb{P}^2$ can contain a Zariski-dense set of integral points (with respect to some nontrivial effective divisor) only if there is a nontrivial completely invariant proper Zariski-closed set with respect to $\phi$. This may be viewed as a generalization of a result of Silverman on integral points in orbits of rational functions. We provide many specific examples, and end with some open problems.