1. Cartan, E.: Sur une classe remaquable d’espaces de Riemann Bull. Soc. Math. France 54, 214–264 (1926)
2. O’Neil, B.: Semi-Riemannian Geometry with Applications to the Relativity. Academic Press, New York (1983)
3. Chaki, M.C., Gupta, B.: On conformally symmetric spaces. Indian J. Math 5, 113–122 (1963)
4. Chaki, M.C.: On pseudo symmetric manifolds. An. Stiint. Univ. Al. I. Cuza Iaşi. Sct. I. a Mat., 33(1), 53–58 (1987)
5. Tamassy, L., Binh, T.Q.: On weakly symmetric and weakly projective symmetric Riemannian manifolds, Differential geometry and its applications, 663–670, Colloq. Math. Soc. Janos Bolyai, 56, North-Holland, Amsterdam (1992)