Abstract. We propose a new point of view of the Spencer cohomology appearing in the formal theory of differential equations based on a dual approach via comodules. It allows us to relate the Spencer cohomology with standard constructions in homological algebra and, in particular, to express it as a Cotor. We discuss concrete methods for its construction based on homological perturbation theory. Appears in Georgian Math. J., vol. 9, No. 4, 2002, 723-772. 1.

doi:10.3842/SIGMA.2009.092 Abstract. A rigorous formulation of Vessiot’s vector field approach to the analysis of general systems of partial differential equations is provided. It is shown that this approach is equivalent to the formal theory of differential equations and that it can be carried through if, and only if, the given system is involutive. As a by-product, we provide a novel characterisation of transversal integral elements via the contact map. Key words: formal integrability; integral element; involution; partial differential equation;

doi:10.3842/SIGMA.2009.092 Abstract. A rigorous formulation of Vessiot’s vector field approach to the analysis of general systems of partial differential equations is provided. It is shown that this approach is equivalent to the formal theory of differential equations and that it can be carried through if, and only if, the given system is involutive. As a by-product, we provide a novel characterisation of transversal integral elements via the contact map. Key words: formal integrability; integral element; involution; partial differential equation; Vessiot connection; Vessiot distribution