1. Hodgkinson, D. E. (1984).Gen. Rel. Grav,16, 569.
2. Eisenhart, L. P. (1947).Riemannian Geometry (Princeton University Press, New Jersey).
3. Szekeres, P. (1966).Nuovo Cim.,43, 1062.
4. Barnes, A. (1974).Gen. Rel. Grav.,5, 147.
5. Debever, R. and Cahen, M. (1960).Bull. Acad. Roy, Belgique Cl. Sci.,47, 491.