References

[1] A. Andreotti, C. D. Hill, Complex characteristic coordinates and tangential Cauchy-Riemann equations, Ann. Scuola Norm. Sup. Pisa (3) 26 (1972), 299–324. MR

[2] R. L. Bryant, S. S. Chern, R. B. Gardner, H. L. Goldschmidt, P. A. Griffiths, Exterior differential systems, Mathematical Sciences Research Institute Publications 18, Springer-Verlag, New York, 1991. MR

[3] R. Bryant, P. Griffiths, L. Hsu, Toward a geometry of differential equations, in Geometry, topology, & physics, Conf. Proc. Lecture Notes Geom. Topology, IV, Int. Press, Cambridge, MA, 1995, pp. 1–76. MR

[4] R. L. Bryant, Real hypersurfaces in unimodular complex surfaces, 2004. arXiv:math/0407472

[5] E. Cartan, Sur les variétés à connexion projective, Bull. Soc. Math. France 52 (1924), 205–241. MR zbM

[6] S. K. Donaldson, Two-forms on four-manifolds and elliptic equations, in Inspired by S. S. Chern, Nankai Tracts Math. 11, World Sci. Publ., Hackensack, NJ, 2006, pp. 153–172. MR

[7] T. A. Ivey, J. M. Landsberg, Cartan for beginners: differential geometry via moving frames and exterior differential systems, Graduate Studies in Mathematics 61, American Mathematical Society, Providence, RI, 2003. MR

[8] V. V. Lychagin, V. N. Rubtsov, The theorems of Sophus Lie for the Monge-Ampère equations, Dokl. Akad. Nauk BSSR 27 (1983), 396–398. MR

[9] L. Nirenberg, On a question of Hans Lewy, Russ. Math. Surv. 29 (1974), 251–262 (English. Russian original).