The degree of irrationality of most abelian surfaces is 4

Abstract

The degree of irrationality of a smooth projective variety $X$ is the minimal degree of a dominant rational map $X⇢{\mathbb{P}}^{\dim X}$. We show that if an abelian surface $A$ over $\mathbb{C}$ is such that the image of the intersection pairing ${\text{Sym}}^{2}NS(A)\to \mathbb{Z}$ does not contain $12$, then it has degree of irrationality $4$. In particular, a very general $(1,d)$-polarized abelian surface has degree of irrationality $4$ provided that $d\nmid 6$. This answers two questions of Yoshihara by providing the first examples of abelian surfaces with degree of irrationality greater than $3$ and showing that the degree of irrationality is not isogeny-invariant for abelian surfaces.

5 pages, comments welcome