1. Gray A., Hervella L.: The sixteen classes of almost Hermitian manifolds and their linear invariants. Ann. Mat. Pura Appl. 123, 35–58 (1980)
2. Gribacheva, D.: Natural connections on Riemannian product manifolds. Comput. Rend. Acad. Bulg. Sci. 64(6), 799–806 (2011)
3. Gribacheva, D.: A natural connection on a basic class of Riemannian product manifolds. Int. J. Geom. Methods Mod. Phys. 9(7), 1250057 (2012)
4. Gribacheva, D., Mekerov, D.: Natural connections on conformal Riemannian P-manifolds. Comput. Rend. Acad. Bulg. Sci. 65(5), 581–590 (2012)
5. Kobayashi, S., Nomizu, K.: Foundations of Differential Geometry, vol.1. Interscience Publishers, New York (1963)