1. A. V. Pogorelov, Extrinsic Geometry of Convex Surfaces (Nauka, Moscow, 1969) [in Russian].
2. A. P. Kopylov, Dokl. Akad. Nauk 410, 21–23 (2006) [Dokl. Math. 74, 637–639 (2006)].
3. A. P. Kopylov, Itogi Nauki Tekh., Ser.: Sovr. Probl. Mat. Fundam. Napravleniya 22, 139–167 (2007).
4. A. P. Kopylov, in Differential Geometry and Its Applications, Proceedings of International Conference DGA2007, Olomouc, Czech Republic, 2007 (World Sci., Singapore, 2008), pp. 157–169.
5. L. Ahlfors and A. Beurling, Acta Math. 83, 101–129 (1950).