Diffieties formalize geometrically the concept of differential equations. We
introduce and study Hamilton-Jacobi diffieties. They are finite dimensional
subdiffieties of a given diffiety and appear to play a special role in the
field theoretic version of the geometric Hamilton-Jacobi theory.Comment: 31 pages, to appear in Journal of Geometry and Physics, slight
changes in the presentation to match the version accepted by the journa
