1. N. I. Achiezer and I. M. Glazman: Theorie der Linearen Operatoren im Hilbert Raum. Akademie-Verlag, Berlin, 1981.
2. F. V. Atkinson: Discrete and continuous boundary problems. Academic Press, New York, 1968.
3. Notas de Algebra y Analisis;A. I. Benedek,1981
4. J. Bognar: Indefinite inner product spaces. Springer-Verlag, Berlin, 1974.
5. E. A. Coddington: Generalized Sturm-Liouville theory. Lectures delivered at the Instructional Conference on Differential Equations, Edinburgh, Scotland, 1967.