Dong Guangchang. (1970). The Semi-global Isometric Imbedding in <em>R</em><sup>3</sup> of Two Dimensional Riemannian Manifolds with Gaussian Curvature Changing Sign Cleanly.
            <i>Journal of Partial Differential Equations</i>. <i>6</i> (1).
            62-79.
         doi: