Ehlers, Jürgen Geometric Analysis and Gravitation, AEI-Golm, MPI for Gravitational Physics, Max Planck Society;
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Ehlers, J., Geren, P., & Sachs, R. K. (1968). Isotropic Solutions of the Einstein-Liouville Equations. Journal of Mathematical Physics, 9(9), 1344-1349. doi:10.1063/1.1664720.