1. I. M. Glazman, Direct Methods of Qualitative Spectral Analysis of Singular Differential Operators, Fizmatgiz, Moscow (1963).
2. F. S. Rofe-Beketov, ?A criterion for the finiteness of the number of discrete levels in a lacuna of the continuous spectrum caused by a perturbation of the periodic potential,? Dokl. AN SSSR,156, No. 3, 515?518 (1964).
3. E. C. Titchmarsh, Eigenfunction Expansions Associated with Second-Order Differential Equations, Vol. II, Oxford Univ. Press, London (1946).
4. N. Rosenfeld, ?The eigenvalues of a class of singular differential operators,? Comm. Pure and Appl. Math.,13, No. 3, 395?405 (1960).