Codimension of Brill-Noether Loci on Abelian Surfaces and the First Segre Invariant

Let $X$ be an abelian surface of Picard rank 1 with ample generator $H$. We show that, for a Mukai vector $\mathbf{v}=(r,mH,\chi )$ with $r,m\ge 1$ and $\chi \le 0$, the first Brill-Noether loci $\{E\mathrm{\mid }{H}^0(E)\mathrm{\ne }0\}$ in $M_X(\mathbf{v})$ is nonempty of the expected codimension $1-\chi$. As an application, we study the rank 1 Segre invariant on these abelian surfaces. Specifically, for $\mathbf{v}$ a Mukai vector of a $\mu$-stable sheaf of $\mathrm{r}\mathrm{a}\mathrm{n}\mathrm{k}\ge 2$, we compute the Segre invariant of the general sheaf in $M_X(\mathbf{v})$, and we determine the numbers which arise as the Segre invariant of a sheaf in $M_X(\mathbf{v})$.