Theta-duality and Prym-Torelli for cyclic covers of hyperelliptic curves

Abstract

Let $f:\tilde{C}\to C$ be an étale cyclic cover of odd prime degree $d$ of a hyperelliptic curve of genus $g\ge 2$. We revisit the theta-duality reconstruction argument for the generic injectivity of the associated Prym map by Naranjo-Ortega-Pirola-Spelta and identify an additional residual locus arising from the fixed divisor of the line bundles occurring in that argument. For $d\ge 5$, this locus does not affect reconstruction, giving injectivity under the numerical assumption $(d-1)(g-1)\ge 7$. For $d=3$, the geometry is governed instead by trigonal pencils on the quotient of $\tilde{C}$ by a lift of the hyperelliptic involution; this yields generic degree $2$ in genus $5$ and injectivity for $g\ge 6$.

arXiv comment14 pages, comments welcome!