Abstract. In order to investigate corrections to the common KdV approximation to long waves, we derive modulation equations for the evolution of long wavelength initial data for a Boussinesq equation. The equations governing the corrections to the KdV approximation are explicitly solvable, and we prove estimates showing that they do indeed give a significantly better approximation than the KdV equation alone. We also present the results of numerical experiments which show that the error estimates we derive are essentially optimal.Key words. Boussinesq equation, KdV approximation, modulation equations, water wave problem
AMS subject classifications. 76B15, 35Q51, 35Q53PII. S11111111024112981. Introduction. Modulation, or amplitude, equations are approximate, often explicitly solvable, model equations derived-usually through asymptotic analysis and the method of multiple time scales-to model more complicated physical situations. Although these equations have been used for over a century, only lately has there been an attempt to rigorously relate solutions of the modulation equations to the original physical problem. [23], the validity of Korteweg-de Vries (KdV) equations as a leading order approximation to the evolution of long wavelength water waves and to a number of other dispersive partial differential equations has been established.While the KdV approximation is extremely useful due to its simplicity and the fact that the KdV equation can be explicitly solved by the inverse scattering transform, both experimentally and numerically one observes departures from the predictions of the KdV equation. Our goal in this paper is to derive modulation equations which govern corrections to the KdV model. In the present paper, we will not work with the full water wave problem but rather will study modulation equations for long wavelength solutions of the Boussinesq equation: