1. C. Möller, in Les Théories Relativistes de la Gravitation, (Centre National de la Recherche Scientifique, Paris, 1962) p. 15; L. Rosenfeld, Nucl. Phys. 40, 353 (1963);
2. A. Peres, Nucl. Phys. 48, 622 (1963);
3. B. S. DeWitt, in General Relativity: An Einstein Centenary Survey, S. W. Hawking and W. Israel eds. (Cambridge University Press, Cambridge, 1979) p. 680; W. Boucher and J. Traschen, Phys. Rev. D 37, 3522 (1988).
4. J. N. Islam, An Introduction to Mathematical Cosmology (Cambridge University Press, Cambridge, 2002); M. Ahmed, S. Dodelson, P. B. Greene, R. Sorkin, e-print astro-ph/0209274.
5. P. Pechukas, Phys. Rev. 181, 166 (1969);