1. E. C. G. Stueckelberg:Helv. Phys. Acta,14, 316 (1941);15, 23 (1941);15, 51 (1941).
2. L. P. Horwitz andC. Piron:Helv. Phys. Acta,46, 316 (1973);L. P. Horwitz andR. Arhanski:J. Phys. A,15, L659 (1982); see alsoF. Ravndal, ref. (3).
3. F. Ravndal:Phys. Rev.,21, 2823 (1980);R. P. Feynman:Phys. Rev.,80, 440 (1950).
4. A. O. Barut andG. H. Mullen:Ann. Phys. (N. Y.),20, 203 (1962);A. O. Barut:Electromagnetic and Classical Theory of Fields and Particles (New York, N. Y., 1964), p. 122.
5. E. Recami andR. Mignani:Riv. Nuovo Cimento,4, 209 (1974), and references therein.