1. Hicks, N. J. (1965).Notes on Differential Geometry. D. van Nostrand Company, Princeton, New Jersey.
2. Busemann, H. (1955).The Geometry of Geodesies. Academic Press, New York.
3. Busemann, H. (1967). Timelike Spaces,Diss. Mathe., 53.
4. Hawking, S. W., and Ellis, G. F. R. (1973).The Large Scale Structure of Space-time. Cambridge University Press, London.
5. Geroch, R. (1968).Ann. Phys.,48, 526.