Laplace-isospectral hyperbolic 2-orbifolds are representation-equivalent

Abstract

Using the Selberg trace formula, we show that for a hyperbolic 2-orbifold, the spectrum of the Laplacian acting on functions determines, and is determined by, the following data: the volume; the total length of the mirror boundary; the number of conepoints of each order, counting a mirror corner as half a conepoint; and the number of primitive closed geodesics of each length and orientability class, counting a geodesic running along the boundary as half orientation-preserving and half orientation-reversing, and discounting imprimitive geodesics appropriately. This implies that Laplace-isospectral hyperbolic 2-orbifolds determine equivalent linear representations of Isom(H^2), and are isospectral for any natural operator.