1. For an extensive discussion of the whole problem, the reader is referred to “Proceedings of Seminar on Unified Theories of Elementary Particles” edited byD. Lurié andN. Mukunda — URPA — 11 July, 1963.
2. See, for example,J. Goldstone, nuovo Cimento,19, 154, 1961;J. Goldstone, A. Salam andS. Weinberg, Phys. Rev.,127, 965, 1962;S. Bludman andA. Klein, Phys. Rev,131, 2364, 1963.
3. A. Klein andB. W. Lee, Phys. Rev. Letters,12, 266, 1964.
4. P. C. Martin, J. Math. Phys., 208, 1963;C. De Dominicis, J. Math. Phys.,4, 255, 1963;P. C. Hohenberg, “Excitations in a dilute Bose gas”, unpublished thesis — Harvard University, 1962;C. De Dominicis andP. C. Martin, J. Math. Phys., 5, 14, 1964 and5, 31, 1964. The author is grateful to ProfessorE. P. Gross for making available to him his own copy ofHohenberg’s work.
5. N. N. Bogoliubov andD. V. Shirkov, “Introduction to the Theory of Quantized Fields”, page 426 — Interscience Publishers Inc., N. Y., 1959.