1. See, for example,R. Haag:Nuovo Cimento,25, 287 (1962);R. F. Streater:Proc. Roy. Soc.,287 A, 510 (1965).
2. M. Weinstein:Chiral symmetry, inSpringer Tracts in Modern Physics,60 (1971).
3. A. R. Dashen:Current Algebras (New York, N. Y., 1968).
4. See, for example,T. W. Kibble:The Goldstone Theorem, Rochester Conference, 1967 (New York, N. Y., 1967).
5. Y. Nambu andG. Jona-Lasinio:Phys. Rev.,122, 345 (1961).