1. L. F. Li and H. Pagels, Phys. Rev. Lett. 27: 1089 (1971).
2. S. Weinberg, Phvsica 96A: 327 (1979).
3. T. N. Truong, Phys. Lett. 99B:228 (1981). For a review, see T.N. Truong in Wandering in the Fields, edited by K. Kawarabayashi and A. Ukawa (World Scientific Singapore, 1987 ).
4. For a review, see: S.L. Adler and R.F. Dashen, “Current Algebra,” W.A. Benjamin, New York (1968).
5. N. I. Muskhelishvili, “Singular Integral Equations,” P. Noordhoff, Gromingen (1953). R. Omnès, Nuovo Cimento 8: 316 (1958).