1. J. A. Wolf, Spaces of Constant Curvature, 5th ed. (Publish or Perish, Boston, Mass., 1981; Nauka, Moscow, 1982).
2. S. Kobayashi and K. Nomizu, Foundations of Differential Geometry, Vols. I and II (Interscience, New York, 1963, 1969; Nauka, Moscow, 1981).
3. H. D. Nguyěñ, Proc. Am. Math. Soc. 128, 3425–3433 (2000).
4. O. Kowalski and L. Vanhecke, Boll. Un. Mat. Ital. B (7) 5(1), 189–246 (1991).
5. H. Tamaru, Osaka J. Math. 36, 835–851 (1999).