A signed count of 2-torsion points on real abelian varieties

Abstract

We prove that a natural signed count of the $2$-torsion points on a real principally polarized abelian variety $A$ always equals to $2^g$ where $g$ is the dimension of $A$. When $A$ is the Jacobian of a real curve we derive signed counts of real odd theta characteristics. These can be interpreted in terms of the extrinsic geometry of contact hyperplanes to the canonical embedding of the curve. We also formulate a conjectural generalization to arbitrary fields in terms of ${\mathbb{A}}^1$-enumerative geometry.