1. B. Simon, Ann. Phys.,97, 279 (1976).
2. J. S. Howland, Pacif. J. Math.,55, 157 (1974).
3. S. N. Lakaev, Dokl. Akad. Nauk Uzb. SSR, No. 4 (1979).
4. I. C. Gohberg and M. G. Krein, Introduction to the Theory of Linear Nonself-adjoint Operators, AMS, Providence, R.I. (1969).
5. V. I. Smirnov, Course of Higher Mathematics, Vol. 4 [in Russian], Fizmatgiz (1958).