1. V. Fock, The theory of Space, Time and Gravitation, 2nd revised Edition (Pergamon Press) 1966.
2. Yu. V. Novozhilov and Yu. A. Yappa, Electrodynamics, Mir, Moscow, 1981.
3. A. Peres and N. Rosen, Nuovo Cimento, XVIII, N.4, 644, 1960.
4. G. A. Bliss, Lectures on the calculus of variations, Univ. Chicago Press, Chicago, 1946; Am. J. Math., 52, 673, 1930.
5. M. R. Hestenes, Am. J. Math., 58, 391, 1936.