1. Einstein, A. (1915).Sitzungsber. Preuss. Akad. Wiss. Berlin, Math.-Phys. Kl., 778.
2. Landau, L. D., and Lifschitz, E. M. (1951).The Classical Theory of Fields (Addison-Wesley, Cambridge).
3. Bergmann, P. G., and Thomson, R. (1953).Phys. Rev.,89, 400.
4. Goldberg, J. N. (1958).Phys. Rev.,111, 315.
5. Schr�dinger, E. (1918).Phys. Z.,19, 4.