Tropical invariants for binary quintics and reduction types of Picard curves

Abstract

In this paper, we express the reduction types of Picard curves in terms of tropical invariants associated to binary quintics. These invariants are connected to Picard modular forms using recent work by Cléry and van der Geer. We furthermore give a general framework for tropical invariants associated to group actions on arbitrary varieties. The previous problem fits in this general framework by mapping the space of binary forms to symmetrized versions of the Deligne-Mumford compactification M 0,n . We conjecture that the techniques introduced here can be used to find tropical invariants for binary forms of any degree.