1. C. N. Yang andJ. Tiomno:Phys. Rev.,79, 495 (1950).
2. B. Zumino andD. Zwanziger: preprint (New York University).
3. P. Carruthers:Local field theory and isospin invariance, I, II; to be published inJourn. Math. Phys.
4. G. C. Wick, A. S. Wightman andE. P. Wigner:Phys. Rev.,88, 101 (1952).
5. See alsoP. Roman:Theory of Elementary Particles, 2nd Ed. (Amsterdam, 1961), p. 262.