The Fekete-Szego theorem with Local Rationality Conditions on Curves

Abstract

Let $K$ be a number field or a function field in one variable over a finite field, and let $K^{sep}$ be a separable closure of $K$. Let $C/K$ be a smooth, complete, connected curve. We prove a strong theorem of Fekete-Szego type for adelic sets $E = \prod_v E_v$ on $C$, showing that under appropriate conditions there are infinitely many points in $C(K^{sep})$ whose conjugates all belong to $E_v$ at each place $v$ of $K$. We give several variants of the theorem, including two for Berkovich curves, and provide examples illustrating the theorem on the projective line, and on elliptic curves, Fermat curves, and modular curves.