___

• [1] Callahan, J. J., Geometry of Spacetime: An Introduction to Special and General Relativity, Springer-Verlag, New York, 2000.
• [2] Chen, B.-Y., Geometry of submanifolds, M. Dekker, New York, 1973.
• [3] Chen, B.-Y., An invariant of conformal mappings, Proc. Amer. Math. Soc., 40(1973), pp. 563–564.
• [4] Chen, B.-Y., Some conformal invariants of submanifolds and their applications, Boll. Un. Mat. Ital., 10(1974), 380–385.
• [5] Chen, B.-Y., Classification of totally umbilical submanifolds in symmetric spaces, J. Austral. Math. Soc. (Series A), 30(1980), 129–136.
• [6] Chen, B.-Y., Total mean curvature and submanifolds of finite type, World Scientific, New Jersey, 1984.
• [7] Chen, B.-Y., Complete classification of spatial surfaces with parallel mean curvature vector in arbitrary non-flat pseudo-Riemannian space forms, Central European J. Math., 7(2009), No.3, pp. 400–428.
• [8] Chen, B.-Y., Pseudo-Riemannian sumbanifolds, δ-invariants and Applications, World Scien- tific, 2011.
• [9] Chen, B.-Y. and Garay, O. J. , Complete classification of quasi-minimal surfaces with parallel mean curvature vector in neutral pseudo-Euclidean 4-space E4, Result. Math., 55(2009),23–38.
• [10] Delaunay, C., Sur la surface de r´evolution dont la courbure moyenne est constante, J. Math. Pure Appl., 6(1841), 309–320.
• [11] Dursun, U., Rotation hypersurfaces in Lorentz-Minkowski space with constant mean curva- ture, Taiwanese J. of Math., 14(2010), No.2, pp. 685–705.
• [12] Dursun, U., Turgay, N. C., Minimal and Pseudo-Umbilical Rotational Surfaces in Euclidean Space E4 Mediterr. J. Math. 10 (2013), no. 1, 497–506.
• [13] Ho, P. T., Remarks on De Sitter Spacetime: Geometry in the Theory of Relativity, Di- mensions, The Journal of Undergraduate Research in Natural Sciences and Mathematics, California State University, Fullerton, 13(2011), pp. 71– 81.
• [14] Mirsky, L., The spread of a matrix, Mathematika, 3(1956), pp. 127–130.
• [15] O’Neill, B., Semi-Riemannian geometry with applications to relativity, Academic Press, San Diego, 1983.
• [16] Spivak, M., A Comprehensive Introduction to Differential Geometry, volume IV, Third edi- tion, Publish or Perish, 1999.
• [17] Suceav˘a, B. D., The spread of the shape operator as conformal invariant, J. Inequal. Pure Appl. Math. 4 (2003), no. 4, Article 74, 8 pp.