1. N. A. Chernikov and N. S. Shavokhina, Teor. Mat. Fiz.,42, 59 (1980).
2. A. P. Norden, Theory of Surfaces [in Russian], GITTL (1956).
3. N. A. Chernikov and N. S. Shavokhina, Preprint R2-10375 [in Russian], JINR, Dubna (1977).
4. W. Blaschke, Introduction to Differential Geometry [Russian translation], Gostekhizdat (1957).