1. Petrov, A. Z. (1954).Sci. Notices of Kazan State University 114, 55.
2. Petrov, A. Z. (1969).Einstein Spaces (Pergamon Press, Oxford).
3. Penrose, R. (1960).Ann. Phys. (NY) 10, 171.
4. Penrose, R., and Rindler, W. (1986).Spinors and Space-Time (Cambridge University Press, Cambridge), vol. 2.
5. See, for example, Kramer, D., Stephani, H., MacCallum, M. A. H., and Herlt, E. (1980).Exact Solutions of Einstein's Field Equations (Cambridge University Press, Cambridge).