1. G. Velo andD. Zwanziger:Phys. Rev.,186, 1337 (1969);188, 2218 (1969).
2. M. Fierz andW. Pauli:Proc. Roy. Soc.,173 A, 211 (1939).
3. For a precise definition seeA. S. Wightman:Proceedings of Fifth Coral Gables Conference (1968).
4. B. Schrorer, A. Swieca andR. Seiler:Phys. Rev. D,2, 2927 (1970).
5. J. Leray andY. Ohya:Systèmes linéaires hyperboliques non-stricts, Colloque C.B.M. (Louvain, 1964) ; reprint inBattelle Rencontres on Hyperbolic Equations and Waves (Seattle, Wis., 1968).