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