On the Prym map for cyclic covers of genus two curves

Abstract

The Prym map assigns to each covering of curves a polarized abelian variety. In the case of étale cyclic covers of curves of genus two, we show that the Prym map is ramified precisely on the locus of bielliptic covers. The key observation is that we can naturally associate to such a cover an abelian surface with a cyclic polarization, and then the codifferential of the Prym map can be interpreted in terms of multiplication of sections on the abelian surface. Furthermore, we prove that a genus two cyclic cover of degree at least seven is never hyperelliptic.