Comparison of solutions of some pairs of nonlinear wave equations

In this thesis, we compare solutions of the Camassa-Holm equation with solutions of the Double Dispersion equation and the Hunter-Saxton equation. In the rst part of this thesis work, we determine a class of Boussinesq-type equations from which can be asymptotically derived. We use an expansion determined by two small positive parameters measuring nonlinear and dispersive e ects. We then rigorously show that solutions of the Camassa-Holm equation are well approximated by corresponding solutions of a certain class of the Double Dispersion equation over a long time scale. Finally we show that any solution of the Double Dispersion equation can be written as the sum of solutions of the two decoupled Camassa-Holm equations moving in opposite directions up to a small error. We observe that the approximation error for the decoupled problem is greater than the approximation error characterized by single Camassa-Holm approximation. We also obtain similar results for Benjamin-Bona-Mahony approximation to the Double Dispersion equation in the long wave limit. In the literature, Hunter- Saxton equation arises as high frequency limit of the Camassa-Holm equation. In the second part of this thesis work, we establish convergence results between the solutions of the Hunter-Saxton equations and the solutions of the Camassa-Holm equation in periodic setting providing a precise estimate for the approximation error