1. For instance, Ellis, G. F. R. (1984). in Proceedings of the Tenth International Conference on General Relativity and Gravitation, B. Bertotti, F. De Felice, and A. Pascolini, eds. (Reidel, Dordrecht); Sato, H. ibid; Futamase, T. (1988) Phys. Rev. Lett.
61, 2175; (1989). Mon. Not. R. astr. Soc.
237, 187.
2. For a concise review of the averaging problem, see Krasiński, A. (1997). Inhomogeneous Cosmological Models (Cambridge University Press, Cambridge), Chapter 8. See also the references therein.
3. Gromov, M., Lafontaine, J., and Pansu, P. (1981). Structures Métriques pour les Variétés Riemannienness (Cedic/Fernand Nathan, Paris).
4. Petersen, P. (1998). Riemannian Geometry (Springer-Verlag, New York), Chapter 10.
5. Seriu, M. (1996). Phys. Rev. D
53, 6902.