1. Rees, M, in Black Holes and Relativistic Stars, Ed by R. (Wald University o Chicago Press, Chicago, 1998), p. 79.
2. Synge, L, in Relativity, the General Theory, (North-Holland Publishing Company, Amsterdam, 1966)
3. A. Papapetrou y A. Hamouni, Ann. Ins. Henri. Poincaré 9, 179–188 (1968)
4. J. Katz, J. Biucák y Lynden-Bell D, Class. Quantum Grav 21, 1–51 (2004)
5. R. Wald, General Relativity. (The University of Chicago Press, Chicago y Londres, 1984)