1. A.L. Besse, Einstein Manifolds, Ergeb. Math. Grenzgeb., 3. Folge, Bd. 10 (Springer-Verlag, Berlin, Heidelberg, New York, 1987)
2. R. Deszcz, M. Głogowska, M. Hotloś, K. Sawicz, in A survey on generalized einstein metric conditions, ed. by S.-T. Yau (series ed.), M. Plaue, A.D. Rendall and M. Scherfner. Advances in Lorentzian geometry: Proceedings of the Lorentzian Geometry Conference in Berlin, AMS/IP Studies in Advanced Mathematics 49, 27–46, 2011
3. Z.I. Szabó, Structure theorems on Riemannian spaces satisfying $$R(X, Y) \cdot R = 0$$ R ( X , Y ) · R = 0 . I. The local version. J. Differ. Geom. 17, 531–582 (1982)
4. R. Deszcz, On pseudosymmetric spaces. Bull. Belg. Math. Soc. Ser. A 44, 1–34 (1992)
5. R. Deszcz, W. Grycak, On some class of warped product manifolds. Bull. Inst. Math. Acad. Sin. 15, 311–322 (1987)