1. Dehnen, H. (1970).International Journal of Theoretical Physics, Vol. 3, No. 6, p. 509.
2. Ehlers, J. and Kundt, W. (1962). InGravitation: an Introduction to Current Research (Ed. L. Witten). J. Wiley, New York, London.
3. Ellis, G. F. R. (1971). InGeneral Relativity and Cosmology (Ed. R. K. Sachs). Academic Press, New York, London.
4. Trautman, A. (1964). InLectures on General Relativity, Section 6.6. Prentice Hall, Englewood Cliffs, New Jersey.