1. Yakupov, M. Sh. (1973).Grav.: Teor. Otnos. Univ. Kazan,9, 109.
2. Barnes, A. (1974).Gen. Rel. Grav.,5, 147.
3. Backhouse, N. (private communication).
4. Kaigorodov, B. R., and Yakupov, M. Sh. (1968).Grav. Teor. Otnosit. Kazan Tern. Shorn, No. 4–5, p. 151–156.
5. Eisenhart,Riemannian Geometry (Princeton University Press, 1947).