Generalization of a formula of Wolpert for balanced geodesic graphs on closed hyperbolic surfaces

A well-known theorem of Wolpert shows that the Weil-Petersson symplectic form on Teichm"uller space, computed on two infinitesimal twists along simple closed geodesics on a fixed hyperbolic surface, equals the sum of the cosines of the intersection angles. We define an infinitesimal deformation starting from a more general object, namely a balanced geodesic graph, by which any tangent vector to Teichm"uller space can be represented. We then prove a generalization of Wolpert’s formula for these deformations. In the case of simple closed curves, we recover the theorem of Wolpert.