1. J. Goldstone:Nuovo Cimento,19, 154 (1961).
2. See, for example,P. G. De Gennes:Superconductivity of Metals and Alloys (New York, Amsterdam, 1966).
3. J. Goldstone, A. Salam andS. Weinberg:Phys. Rev.,127, 965 (1962).
4. P. De Mottoni andE. Fabri:Nuovo Cimento,54 A, 42 (1968).
5. D. Kastler:Proceedings of the International Conference on Particles and Fields, Rochester, N. Y., 1967 (New York, London, Sydney, 1967), p. 305;C. A. Orzalezi:Rev. Mod. Phys.,42, 381 (1970).