Invariant Modules and the Reduction of Nonlinear Partial Differential Equations to Dynamical Systems

We completely characterize all nonlinear partial differential equations leaving a given finite-dimensional vector space of analytic functions invariant. Existence of an invariant subspace leads to a reduction of the associated dynamical partial differential equations to a system of ordinary differential equations, and provide a nonlinear counterpart to quasi-exactly solvable quantum Hamiltonians. These results rely on a useful extension of the classical Wronskian determinant condition for linear independence of functions. In addition, new approaches to the characterization of the annihilating differential operators for spaces of analytic functions are presented. † Supported in part by NSERC Grant OGP0105490. ‡ Supported in part by an NSERC Postdoctoral Fellowship. § Supported in part by NSF Grant DMS 01–03944.