1. R. L. Ingraham:Proc. Nat. Acad. Sci.,41, 165 (1955);Phys. Rev.,101, 1411 (1956). AlsoH. J. Groenewold:Physica,6, 115 (1939);A. Landé:Phys. Rev.,76, 1176 (1949);77, 814 (1950).
2. P. A. M. Dirac:Proc. Roy. Soc., A167, 148 (1938);F. Rohrlich:Classical Charged Particles, Chap. 6 (Reading, Mass., 1965);A. Baiquni:Phys. Rev. D,9, 1084 (1974).
3. This is known as Liouville’s theorem. SeeF. Klein:Vorlesungen über höhere Geometrie, 3rd ed., Sect.50 (Berlin, 1926). This sort of algebraic geometry is very neglected nowadays.
4. R. L. Ingraham:Lett. Nuovo Cimento,7, 846 (1973).
5. There does not exist a point-particle solution corresponding to aC-invariant point-form current on the right-hand side of (2a) (to be published). Moreover, an inhomogeneous form of (2a) with source currentj a on the right would seem unphysical since the current already appears viaF 5μ, cf. (3a).