On the integrality of Seshadri constants of abelian surfaces

Abstract

In this paper we consider the question of when Seshadri constants on abelian surfaces are integers. Our first result concerns self-products $E\times E$ of elliptic curves: If $E$ has complex multiplication in $\mathbb{Z}[i]$ or in $\mathbb{Z}[\frac{1}{2}(1+i\sqrt{3})]$ or if $E$ has no complex multiplication at all, then it is known that for every ample line bundle $L$ on $E\times E$, the Seshadri constant $\eps(L)$ is an integer. We show that, contrary to what one might expect, these are in fact the only elliptic curves for which this integrality statement holds. Our second result answers the question how -- on any abelian surface~-- integrality of Seshadri constants is related to elliptic curves.