1. Atkinson, F. V.: Discrete and Continuous Boundary Problems, Academic Press, New York, 1964.
2. Dunford, N. and Schwartz, J. T.: Linear Operators, Part II, Interscience, New York, 1964.
3. Everitt, W. N. and Halvorsen, S. G.: On the asymptotic form of the Titchmarsh?Weyl m-coefficient, Appl. Anal. 8 (1978), 153?169.
4. Everitt, W. N., Knowles, I. W. and Read, T. T.: Limit-point and limit-circle criteria for Sturm?Liouville equations with intermittently negative principal coefficient, Proc. Roy. Soc. Edinburgh Sect. A 103 (1986), 215?228.
5. Ismagilov, R. S.: Conditions for the semiboundness and discreteness of the spectrum for one-dimensional differential equations, Dokl. Akad. Nauk. SSSR 140 (1961), 33?36; English transl. Soviet Math. Dokl. 2 (1961), 1137?1140.