1. R. Delbourgo, M. A. Rashid, A. Salam andJ. Strathdee:High-Energy Physics and Elementary Particles (Vienna, 1965) p. 455;J. Daboul andR. Delbourgo:Nuovo Cimento,44 A, 1031 (1966).
2. For references seeA. Tavkhelidze:High-Energy Physics and Elementary Particles (Vienna, 1965), p. 763.
3. J. Harte: CERN preprint, which contains an analysis along these lines.
4. R. E. Cutkosky:Phys. Rev.,96, 1135 (1954).
5. This particular differential equation was first derived in ref. (4). Our method is different and in further Sections is generalized to the spinor case.