1. Abrikosov, A.A., Gor'kov, L.P.: Zh. Eksp. Teor. Fiz.39, 1781 (1960)
2. Shiba, H.: Prog. Theor. Phys.40, 435 (1968); Prog. Theor. Phys.50, 50 (1973)
3. Rusinov, A.I.: Zh. Eksp. Teor. Fiz.56, 2047 (1969)
4. For a review see Müller-Hartmann, E.: In: Magnetism. Suhl, H. (ed.), Vol. V. New York, London: Academic Press 1973
5. Ginsberg, D.M.: Phys. Rev. B20, 960 (1979)