1. Aronszajn, N.: On a problem of Weyl in the theory of singular Sturm-Liouville-equations. Amer. J. Math.79, 597?610 (1957).
2. Berkowitz, J.: On the discreteness of spectra of singular Sturm-Liouville Problems. Comm. Pure Appl. Math.12, 513?542 (1959).
3. Birman, M. S.: Über das Spektrum singulärer Randwertprobleme [russisch]. Mat. sb. (N. S.)9, 125?174 (1961).
4. Coddington, E. A., andN. Levinson: Theory of ordinary differential equations. New York-Toronto-London: McGraw-Hill Book Co. 1955.
5. Dunford, N., andJ. T. Schwartz: Linear operators, Part II: Spectral theory. New York and London: Interscience Publishers 1963.