1. C. Carathéodory, Variationsrechnung und partielle Differentialgleichungen erster Ordnung, Teubner, Leipzig, 1935.
2. Some theorems on the isometric imbedding of compact Riemann manifolds in euclidean space;Chern, Shiing-shen;Ann. of Math. (2),1952
3. On the total curvature of immersed manifolds;Chern, Shiing-shen;Amer. J. Math.,1957
4. On spherical image maps whose Jacobians do not change sign;Hartman, Philip;Amer. J. Math.,1959
5. Isometric immersion of flat Riemannian manifolds in Euclidean space;O’Neill, Barrett;Michigan Math. J.,1962