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2) D. J. Scalapino: in Superconductivity, ed. R. D. Parks (Dekker, New York, 1969) Vol. 1, p. 449.
3) P. B. Allen and B. Mitrovi\\'{c}: in Solid State Physics, ed. H. Ehrenreich, F. Seitz and D. Turnbull (Academic, New York, 1982) Vol. 37, p. 1.
4) D. Rainer: in Progress in Low Temperature Physics, ed. D. F. Brewer (North-Holland, Amsterdam, 1986) Vol. 10, p. 371.
5) A. B. Migdal: Zh. Eksp. Teor. Fiz. 34 (1958) 1438. translation: Sov. Phys.-JETP 7 (1958) 996.