Ehlers, Jürgen Geometric Analysis and Gravitation, AEI-Golm, MPI for Gravitational Physics, Max Planck Society;
Ehlers, J., & Schneider, P. (1993). Gravitational Lensing. In Gleiser, R. J., & al. (Eds.), General Relativity and Gravitation: Proceedings of GR 13 (pp. 21-40). Bristol: Institut of Physics Publishing.