1. Newman, E., Penrose, R.: J. Math. Phys. 3, 566 (1962)
2. Ehlers, J.: Akad. Wiss. Lit. Mainz, Abhandl. Math. Nat. Kl. 11, 793 (1961). [Translation: J. Ehlers, Gen. Relativ. Gravit. 25, 1225 (1993)]
3. Ellis, G.F.R.: Relativistic cosmology. In: Proceedings of the International School of Physics “Enrico Fermi”, Course 47: General Relativity and Cosmology. Academic Press, New York (1971)
4. Clarkson, C.A., Barrett, R.K.: Class. Quantum Gravity 20, 3855 (2003)
5. Dyer, C.C., McVittie, G.C., Oattes, L.M.: Gen. Relativ. Gravit. 19, 887 (1987)