On the discreteness of the spectrum of the Laplacian on noncompact Riemannian manifolds

Necessary and sufficient conditions for the discreteness of the Laplacian on a noncompact Riemannian manifold M are estab- lished in terms of the isocapacitary function of M. The relevant capacity takes a different form according to whether M has fi- nite or infinite volume. Conditions involving the more standard isoperimetric function of M can also be derived, but they are only sufficient in general, as we demonstrate by concrete examples. The research that led to the present paper was partially supported by a grant of the group GNAMPA of INdAM.