Ehlers, Jürgen Geometric Analysis and Gravitation, AEI-Golm, MPI for Gravitational Physics, Max Planck Society;
Ehlers, J. (1960). Exterior solutions of Einsteins’s Gravitational Field Equations admitting a two-dimensional Abelian Group of Isometric Correspondences. In Col. Sur la theorie de la relativite (pp. 49-57). Paris: Gauthier-Villar.