Cs. Gordon et Dl. Webb, ISOSPECTRAL CONVEX DOMAINS IN THE HYPERBOLIC PLANE, Proceedings of the American Mathematical Society, 120(3), 1994, pp. 981-983
We construct pairs of nonisometric convex polygons in the hyperbolic p
lane for which the Laplacians are both Dirichlet and Neumann isospectr
al. We also give examples of pairs of isospectral potentials for the S
chrodinger operator on certain convex hyperbolic polygons.