Hyperelliptic genus 3 curves with involutions and a Prym map

Abstract

We characterize genus 3 complex smooth hyperelliptic curves that admit two additional involutions as curves that can be built from five points in $P^1$ with a distinguished triple. We are able to write down explicit equations for the curves and all their quotient curves. We show that, fixing one of the elliptic quotient curve, the Prym map becomes a 2:1 map and therefore the hyperelliptic Klein Prym map, constructed recently by the first author with A. Ortega, is also 2:1 in this case. As a by-product we show an explicit family of $(1,d)$ polarized abelian surfaces (for $d>1$), such that any surface in the family satisfying a certain explicit condition is abstractly non-isomorphic to its dual abelian surface.