Ehlers, Jürgen Geometric Analysis and Gravitation, AEI-Golm, MPI for Gravitational Physics, Max Planck Society;
Ehlers, J., & Rindler, W. (1989). A Novel Phase-Space Representation and Classification of All Friedman-Lemaitre Universes. Annals of the New York Academy of Sciences, 571. doi:10.1111/j.1749-6632.1989.tb50496.x.