1. [GH] M. Gage and R. Hamilton, The heat equation shrinking convex plane curves, J. Differential Geom. 23 (1986), no. 1, 69–96.
2. [Gr] M. Grayson, The heat equation shrinks embedded plane curves to round points, J. Differential Geom. 26 (1987), no. 2, 285–314.
3. [Bl] G. Bluman, An overview of potential symmetries, Exploiting symmetry in applied and numerical analysis (Fort Collins, CO, 1992), Lectures in Appl. Math., vol. 29, Amer. Math. Soc., Providence, RI, 1993, pp. 97–109.
4. [BG] R. Bryant and P. Griffiths, Characteristic cohomology of differential systems (I): General theory, to appear in the J. Amer. Math. Soc., 1995.
5. [BCG3] R. Bryant, S.-S. Chern, R. Gardner, H. Goldschmidt, and P. Griffiths, Exterior differential systems, Mathematical Sciences Research Institute Publications, vol. 18, Springer-Verlag, New York, 1991.