1. S. Albeverio, F. Gesztesy, R. Hoegh-Krohn, and H. Holden, Solvable Models in Quantum Mechanics (AMS Chelsea, Providence, RI, 2005).
2. A. Zettl, Sturm–Liouville Theory, Vol. 121 of Mathematical Survey and Monographs (Am. Math. Soc., USA, 2005).
3. J. von Neumann, ‘‘Allgemeine Eigenwerttheorie hermitescher Funktionaloperatoren,’’ Math. Ann. 102, 49–131 (1929–1930).
4. M. H. Stone, ‘‘Linear transformation in Hilbert space and their applications in analysis,’’ Am. Math. Soc. Collag., № 15, 49–131 (1932).
5. I. M. Glazman, ‘‘On the theory of singular differential operators,’’ Usp. Mat. Nauk 40, 102–135 (1950).