1. Friedrichs, K.O.: Symmetric positive linear differential equations, Comm. Pure Appl Math 11, 333–418 (1956)
2. Günther, M.: Isometric embeddings of Riemannian manifolds, Proceeding of the International Congress of Mathematicians, Kyoto, Japan, 1137–1143 (1990)
3. Han, Q.: On the isometric embedding of surfaces with Gauss curvature changing sign clearly. Comm. Pure Appl. Math. 58, 285–295 (2005) Local isometric embedding of surfaces in ℝ3, Lecture Notes, Max-Planck Institute for Mathematics, Leipzig, (2003)
4. Han, Q., Hong, J.-X., Lin, C.-S.: Local isometric embedding of surfaces with nonpositive gaussian curvature, preprint (2002)
5. Hong, J.-X.: Cauchy problem for degenerate hyperbolic Monge-Ampere equations, J. Partial Diff. Equations 4, 1–18 (1991)