Nonlinear systems related to an arbitrary space–time dependence of the spectral transform

We propose a general algebraic analytic scheme for the spectral transform of solutions of nonlinear evolution equations. This allows us to give the general integrable evolution corresponding to an arbitrary time and space dependence of the spectral transform (in general nonlinear and with non-analytic dispersion relations). The main theorem is that the compatibility conditions gives always a true nonlinear evolution because it can always be written as an identity between polynomials in the spectral variable $k$. This general result is then used to obtain first a method to generate a new class of solutions to the nonlinear Schroedinger equation, and second to construct the spectral transform theory for solving initial-boundary value problems for resonant wave-coupling processes (like self-induced transparency in two-level media, or stimulated Brillouin scattering of plasma waves or else stimulated Raman scattering in nonlinear optics etc...).Comment: 27 pages, Latex file, Submitted to Journ Math Phy