A Hurwitz divisor on the Moduli of Prym curves

Abstract

For genus $g=2i\geq4$ and the length $g-1$ partition $\mu = (4,2,\ldots,2,-2,\ldots,-2)$ of 0, we compute the first coefficients of the class of $\overline{D}(\mu)$ in $\mathrm{Pic}_\mathbb{Q}(\overline{\mathcal{R}}_g)$, where $D(\mu)$ is the divisor consisting of pairs $[C,\eta]\in \mathcal{R}_g$ with $\eta \cong \mathcal{O}_C(2x_1+x_2+\cdots + x_{i-1}-x_i-\cdots-x_{2i-1})$ for some points $x_1,\ldots, x_{2i-1}$ on $C$. We further provide several enumerative results that will be used for this computation.