1. A. A. Abrikosov and L. P. Gorkov,Zh. Eksperim. i Teor. Fiz. 39, 1781 (1960) [English transl.,Soviet Phys.?JETP 12, 1243 (1961)].
2. E. Bucher, K. Andres, J. P. Maita, and G. W. Hull, Jr.,Helv. Phys. Acta 41, 723 (1968).
3. J. Cooper, Ph.D. thesis, Imperial College, London, 1969, and B. Coles, private communication.
4. P. Fulde, L. L. Hirst, and A. Luther,Z. Physik 230, 155 (1970).
5. P. Fulde and H. E. Hoenig,Solid State Commun. 8, 341 (1970).