1. C. Carathéodory, “Über die Variationsrechnung bei mehrfachen Integralen,” Acta Szeged 4, 193–216 (1929).
2. W. K. Clifford, “On a surface of zero curvature and finite extent,” Proc. London Math. Soc. 4, 381–395 (1873).
3. Yu. S. Osipov and M. I. Zelikin, “Multidimensional generalization of Jacobi envelope theorem,” Russian J. Math. Phys. 19 (1), 101–106 (2012). doi https://doi.org/10.1134/S1061920812010086
4. M. M. Postnikov, Lectures in Geometry: Lie Groups and Lie Algebras. Semester V (Nauka, Moscow, 1982) [in Russian].
5. J. Simons, “Minimal varieties in Riemannian manifolds,” Ann. Math. 88, 62–105 (1969).