Laplacian spectrum ($\mathrm{Spec}(\Delta )$)

The spectrum of a specified self-adjoint Laplace-Beltrami operator. On a closed manifold of positive dimension, it is a discrete sequence of nonnegative eigenvalues tending to infinity, counted with the dimensions of their Laplacian eigenspaces. On a noncompact manifold or a manifold with boundary, the domain and boundary conditions are part of the operator definition and continuous spectrum can occur.