35Pxx Spectral theory and eigenvalue problems for partial

35P05  General spectral theory of PDE
35P10  Completeness of eigenfunctions, eigenfunction expansions
               for PDO
35P15  Estimation of eigenvalues, upper and lower bounds
35P20  Asymptotic distribution of eigenvalues and eigenfunctions
               for PDO
35P25  Scattering theory for PDE, See also {47A40}
35P30  Nonlinear eigenvalue problems, nonlinear spectral theory
               for PDO
35P99  None of the above but in this section