Local Rigidity of Triangle Groups in Sp(4,R)

Abstract

This paper studies configurations of Lagrangians in a four dimensional real symplectic vector space. We develop a generalized cross ratio as an invariant for quadruples of Lagrangians. This invariant is then used to study representations of triangle groups into the symplectic group. The main theorem is a local rigidity result for a certain representations factoring through the isometry group of the hyperbolic plane.