1. J. Saks and K. Uhlenbeck, Ann. Math. 113 (1), 1–24 (1981).
2. K. Kenmotsu and C. Xia, Kodai Math. J. 18, 242–249 (1995).
3. A. A. Borisenko, Intrinsic and Extrinsic Geometry of Multidimensional Submanifolds (Ekzamen, Moscow, 2003) [in Russian].
4. A. Preissmann, Comment. Math. Helv. 15, 175–216 (1943).
5. M. Berger, Comp. Rend. Acad. Sci. Paris 257 (26), 4122–4125 (1963).