1. For an excellent discussion of and references on symmetries and spontaneously broken symmetries, see
2. S. Weinberg, Brandeis Lectures, 1970;
3. M. A. B. Beg, Lectures Notes in Mexico, 1971;
4. G. Guralnik, C. R. Hagen, and T. W. B. Kibble, Advances in High-Energy Physics (edited by R. Cool and R. E. Marshak ), Wiley, New York, 1969.
5. R. Gatto, A Basic Course in Modern Weak Interaction Theory,Bologna preprint (1979) (unpublished).