1. See for instance R. Brout “phase transitions”, W.A. Benjamin Inc. New York NY, (1963).
2. Y. Nambu, Phys.Rev.Lett. 4, 380 (1960).
3. Y. Nambu and G. Jona-Lasinio, Phys.Rev. 122, 345 (1961).
4. F. Englert and R. Brout, Phys.Rev. 13, 321 (1964).
5. P.W. Biggs, Phys.Lett. 12, 132 (1964); Phys.Rev.Lett. 13, 508 (1964); Phys.Rev. 145, 1156 (1966).