1. J. D. Kimel andL. M. Nath:Phys. Rev. D,6, 2132 (1972).
2. An existence proof for the Green’s function of the external-field Rarita-Schwinger equation is obviously very desirable, but does not seem to be attempted in the literature. The excellent review article:A. S. Wightman:Relativistic wave equations as singular hyperbolic systems, inProceedings of Symposia in Pure Mathematics, Vol.23, edited byD. C. Spencer, American Mathematical Society, reports on what mathematical results are at hand. The methods used in ref. (3–5)G. Velo andD. Zwanziger:Phys. Rev.,186, 1337 (1969).G. Velo:Nucl. Phys.,43 B, 389 (1972).G. Velo andD. Zwanziger: inLectures from the Coral Gables Conference on Fundamental Interactions at High Energy, Vol.4, edited byM. Dal Cin, G. J. Iverson andA. Perlmutter (New York, N. Y., London, and Paris, 1971), p. 8;G. Velo:Lett. Nuovo Cimento,3, 80 (1972);
3. A. Shamaly andA. Z. Capri:Lett. Nuovo Cimento,3, 467, 637 (1972), give properties of the solutions, assuming their existence. If, contrary to our expectations, no solutions exist, the fallacy of perturbative methods is clear.
4. G. Velo andD. Zwanziger:Phys. Rev.,186, 1337 (1969).
5. G. Velo:Nucl. Phys.,43 B, 389 (1972).