1. F. M. Morse and G. Feshbach, Methods of Theoretical Physics (McGraw-Hill, New York, 1953; Inostr. Liter., Moscow, 1960), Vol. 2 [in Russian].
2. P. D. Lax and R. S. Philips, Scattering Theory (Academic, New York, 1967; Mir, Moscow, 1971).
3. M. Reed and B. Simon, Methods of Modern Mathematical Physics (Academic, New York, 1975; Mir, Moscow, 1982).
4. C. Müller, J. Rat. Mech. Anal. 4, 235 (1955).
5. A. B. Migdal, Qualitative Methods in Quantum Theory (Nauka, Moscow, 1975; Benjamin, Reading, Mass., 1977).