1. E. C. Titchmarsh, Eigenfunction Expansions Associated with Second-Order Differential Equations, Pt. 1, Oxford Univ. Press (1962).
2. B. M. Levitan and I. S. Sargsyan, Introduction to Spectral Theory [in Russian], Nauka, Moscow (1970).
3. M. A. Naimark, Linear Differential Operators [in Russian], Nauka, Moscow (1969).
4. I. M. Glazman, Direct Methods of Qualatative Spectral Analysis of Singular Differential Operators [in Russian], Fizmatgiz, Moscow (1963).
5. J. L. Lions, Equations Différentielles Opérationelles et Problemes aux Limites, Springer (1961).