1. Brinkmann, H. W. (1925). Math. Ann.94, 119.
2. Ehlers, J., and Kundt, W. (1962). In Gravitation: An Introduction to Current Research, L. Witten, ed. (Wiley, New York).
3. Daftardar, V., and Dadhich, N. (1994). Gen. Rel. Grav.26, 859, and the references therein.
4. Choquet-Bruhat, Y., DeWitt-Morette, C., and Dillard-Bleik, M. (1991). Analysis Manifolds and Physics: Basics(North-Holland, Amsterdam).
5. Stephani, H. (1990). In General Relativity(2nd edition, Cambridge University Press, Cambridge).