1. H. Weyl:Z. Phys.,56, 330 (1929).
2. P. A.M. Dirac:Proc. R. Soc. London, Ser. A,133, 60 (1931).
3. M. H. Saffouri:Nuovo Cimento D,3, 589 (1984);J. Phys. A,16, 4377 (1983).
4. J. D. Jackson:Classical Electrodynamics, 2nd Edition (John Wiley and Sons Inc., New York, N. Y., 1975). On p. 219 Jackson writes: « But it is often convenient to introduce potentials, obtaining a smaller number of second-order equations, while satisfying some of the Maxwell equations identically ».
5. M. Jammer:Concepts of Force (Harvard University Press, Cambridge, Mass., 1957), Chapt. 11.