Adapted hyperbolic polygons and symplectic representations for group actions on Riemann surfaces

Abstract

We prove that given a finite group G together with a set of fixed geometric generators, there is a family of special hyperbolic polygons that uniformize the Riemann surfaces admitting the action of G with the given geometric generators. From these special polygons, we obtain geometric information for the action: a basis for the homology group of surfaces, its intersection matrix, and the action of the given generators of G on this basis. We then use the Frobenius algorithm to obtain a symplectic representation g. of G corresponding to this action. The fixed point set of g, in the Siegel upper half-space corresponds to a component of the singular locus of the moduli space of principally polarized abelian varieties. We also describe an implementation of the algorithm using the open source computer algebra system SAGE.