Ehlers, Jürgen Geometric Analysis and Gravitation, AEI-Golm, MPI for Gravitational Physics, Max Planck Society;
Ehlers, J. (1983). Relations between the Galilei-invariant and the Lorentz-invariant theories of collisions. In D. Mayr, & G. Samann (Eds.), Space, Time and Mechanics (pp. 21-37). Dordrecht: Reidel.