1. A. Martin:Nuovo Cimento,59 A, 131 (1969).
2. R. G. Newton:Journ. Math. Phys.,9, 2050 (1968).
3. I. A. Sakmar:Lett. Nuovo Cimento,2, 256 (1969).
4. H. Goldberg:Phys. Rev. D,1, 1242 (1970).
5. See, for instance,L. I. Schiff:Quantum Mechanics (New York, N. Y., 1968), p. 140. (See alsoS. S. Schweber, H. A. Bethe andF. de Hoffmann:Mesons and Fields, Vol.1 (Evanston, Ill., 1956), p. 42, 74 and also p. 309.)