1. Petrov, A. Z. (1969).Einstein Spaces (Pergamon Press, New York).
2. Kinnersley, W. (1969).J. Math. Phys.,10, 1195–1203.
3. Ehlers, J., and Kundt, W. (1962). InGravitation: An Introduction to Current Research, Witten, L., ed. (Wiley, New York).
4. Kinnersley, W., and Walker, M. (1970).Phys. Rev. D,2, 1359–1370.
5. Farhoosh, H., and Zimmerman, R. L. (1980).Phys. Rev. D,21, 317–327.