Covering of Elliptic Curves and the Kernel of the Prym Map

  • Sara Torelli University of Pavia
  • Filippo Francesco Favale University of Trento


Motivated by a conjecture of Xiao, we study families of coverings of elliptic curves and their corresponding Prym map . More precisely, we describe the codi↵erential of the period map P associated to in terms of the residue of meromorphic 1-forms and then we use it to give a characterization for the coverings for which the dimension of Ker(dP) is the least possibile. This is useful in order to exclude the existence of non isotrivial fibrations with maximal relative irregularity and thus also in order to give counterexamples to the Xiao’s conjecture mentioned above. The first counterexample to the original conjecture, due to Pirola, is then analysed in our framework.