Characterization of eigenvalues in spectral gap for singular differential operators (Q696031)

The authors retell for symmetric differential operators the well-known results by M.\,G.\thinspace Krein about eigenvalues of selfadjoint extensions of a symmetric operator with a spectral gap; see [\textit{M. Krein}, Mat. Sb., N. Ser. 20(62), 431--495 (1947; Zbl 0029.14103)]. The reference to Krein's paper is given, and the general result covering the main theorem of the paper under review is even mentioned in the introduction.