1. Cartan, E. (1946).LeÇons sur la Geometrie des Espaces de Riemann (Gauthier-Villars, Paris).
2. Thomas, T. Y. (1934).The Differential Invariants of Generalized Spaces (Cambridge U.P., Cambridge).
3. Brans, C. H. (1965).J. Math. Phys.,6, 94.
4. Ehlers, J., and Kundt, W. (1962). InGravitation: An Introduction to Current Research, Ed. L. Witten (Wiley, New York).
5. Misner, C. W., Thorne, K. S., and Wheeler, J. A. (1972),Gravitation (Freeman, San Francisco).