1. N. I. Akhiezer and I. M. Glazman, “Theory of Linear Operators in Hilbert Space” (translated by M. Nestell), Frederick Ungar Publishing Co., New York (1963).
2. F. Riesz and B. Sz. Nagy, “Functional Analysis” (translated by L. Boron), Frederick Ungar Publishing Co., New York (1955).
3. T. Kato, “Perturbation Theory for Linear Operators”, Springer-Verlag, N.Y. (1966).
4. M. Schechter, “Principles of Functional Analysis”, Academic Press, N.Y. (1971).
5. N. Dunford and J. Schwartz, “Linear Operators”, Interscience, vol. I (1958) and II (1963).