1. Ferraro, V. C. A.: Electromagnetic theory. London: Athlone Press, University of London 1962.
2. Gautreau, R., andJ. L. Anderson: Physics Letters25 A, No. 4, 291 (1967).
3. Israel, W.: Il Nuovo Cimento, Serie X,44, 1 (1966).
4. Sygne, J. L.: Relativity — the general theory. Amsterdam: North Holland Publishing Company 1960.
5. Whittaker, E. T.: Proceedings of the Royal Society A.149, 384 (1935).