1. I. Anderson and N. Kamran.The variational bicomplex for second order partial differential equations in the plane I: generalized Laplace invariants and their applications. preprint, Fall 1993.
2. R. Bryant, S.-S. Chern, R. Gardner, H. Goldschmidt, and P. Griffiths.Exterior differential systems. Mathematical Sciences Research Institute Publications #18, Springer-Verlag (1991).
3. R. Bryant and P. Griffiths.Characteristic cohomology of differential systems (I): General theory. IAS preprint (1993).
4. R. Bryant and P. Griffiths.Characteristic cohomology of differential systems (II): Conservation Laws for a Class of Parabolic Equations. IAS preprint (1993).
5. R. Courant and D. Hilbert.Methods of Mathematical Physics, Volume I. Interscience, New York (1953),Volume II, Interscience, New York (1962).