1. I. M. Glazman, Direct Methods of Qualitative Spectral Analysis of Singular Differential Operators [in Russian], Gostekhizdat, Moscow (1963).
2. A. M. Molchanov, "On conditions for a discrete spectrum in the case of self-adjoint differential operators of second order," Trudy Mosk. Matem. Obshch.,2, 169?200 (1953).
3. E. Hille and P. Phillips, Functional Analysis and Semigroups [Russian translation], IL, Moscow (1962). [Colloq. Publ. Amer. Math. Soc., 1957.]
4. L. A. Lyusternik and V. I. Sobolev, Elements of Functional Analysis [in Russian], "Nauka," Moscow (1965).
5. F. Riesz and B. Sz.-Nagy, Functional Analysis [Russian translation], IL, Moscow (1954).