1. Exterior Differential Systems;Bryant,2013
2. Characteristic cohomology of differential systems II: Conservation laws for a class of parabolic equations;Bryant;Duke Math. J.,1995
3. Exterior differential systems and Euler-Lagrange partial differential equations;Bryant,2003
4. Toward a geometry of differential equations;Bryant,1995
5. The Method of Equivalence and its Applications;Gardner,1989