1. E. Hille and R. Phillips, Functional Analysis and Semi-Groups (Amer. Math. Soc., Providence, RI, 1957).
2. W. Rudin, Principles of Mathematical Analysis (McGraw-Hill, New York, 1973).
3. T. Kato, Perturbation Theory for Linear Operators (Springer- Verlag, Heidelberg, 1966).
4. S. P. Strelkov, Introduction to Oscillation Theory (Nauka, Moscow, 1964) [in Russian].
5. Yu. L. Daletsky and M. G. Krein, Stability of Solutions of Differential Equations in a Banach Space (Nauka, Moscow, 1970) [in Russian].