QUASI-CLASSICAL APPROACH TO THE INVERSE SCATTERING PROBLEM FOR THE KDV EQUATION, AND SOLUTION OF THE WHITHAM MODULATION EQUATIONS

We consider an initial value problem for the KdV equation in the limit of weak dispersion. This model describes the formation and evolution in time of a nondissipative shock wave in plasma. Using the perturbati on theory in power series of a small dispersion parameter, we arrive a t the Riemann simple wave equation. Once the simple wave is overturned , we arrive at the system of Whitham modulation equations that describ es the evolution of the resulting nondissipative shock wave. The idea of the approach developed in this paper is to study the asymptotic beh avior of the exact solution in the limit of weak dispersion, using the solution given by the inverse scattering problem technique. In the st udy of the problem, we use the WKB approach to the direct scattering p roblem and use the formulas for the exact multisoliton solution of the inverse scattering problem. By passing to the limit, we obtain a fini te set of relations that connects the space-time parameters x, t and t he modulation parameters of the nondissipative shock wave.