1. W.A. Little, Phys. Rev134, A1416 (1964).
2. J. Bardeen, L. Cooper, and J.R. Schrieffer, Phys, Rev. 108, 1175 (1957)
3. N. Bogoluybov, N. Tomachev, and D. Shirkov, A new method in the theory of superconductivity. Cons. Bureau, New York (1959).
4. L. Gor’kov, Sov. Phys. - JETP, 7, 505 (1958)
5. P. de Gennes, Superconductivity in Metals and Alloys, Benjamin, NY (1966).