1. Landau, L. D., and Lifschitz, E. M. (1962)The Classical Theory of Fields (Addison-Wesley, Reading, Mass.) [also reprinted (1975). (Pergamon Press, Oxford)].
2. Lichnerowicz, A. (1955).Théories relativistes de la gravitation et de l'électromagnétisme (Masson, Paris).
3. Synge, J. L. (1937).Proc. Lond. Math. Soc. 43.
4. Salzman, G., and Taub, A. H. (1954).Phys. Rev. 95, 1659.
5. Jordan, P., Ehlers, J., Sachs, R. K. (1961).Akad. Wiss. Mainz. Abh. math.-nat. Kl. 1961 Nr. 1.