1. Tolman, R.C. (1958).Relativity, Thermodynamics and Cosmology, (Oxford University Press), chapter 6.
2. Landau, L.D. and Lifshitz, D.M. (1962).The Classical Theory of Fields, (Addison-Wesley Publishing Co., Inc., Reading, Massachusetts), chapter 11.
3. Reichenbach, H. (1956).The Philosophy of Space and Time, (Dover Publications, Inc., New York), chapter 1.
4. Eisenhart, L.P. (1960).Riemannian Geometry, (Princeton University Press, Princeton, New Jersey), p. 89.
5. Dicke, R.H. (1960).Am. J. Phys.,28, 344.