1. Anderson, P. W., Werthamer, N. R., Luttinger, J. M. (1965).Physical Review,138A, 1157.
2. Evans, W. A. B., and Rickayzen, G. (1964).Proceedings of the Physical Society,83, 311.
3. Kuper, C. G. (1964).Introduction to the Theory of Superconductivity. Oxford Univ. Press, London. Sections 1.2 and 4.3 contain a discussion of the problem.
4. Miller, P. B. (1961).Physical Review,121, 435.
5. Rickayzen, G. (1965).Theory of Superconductivity. John Wiley and Sons, Inc., New York.