Subharmonic dynamics of wave trains in the Korteweg-de Vries/Kuramoto-Sivashinsky equation

We study the stability and nonlinear local dynamics of spectrally stable periodic wave trains of the Korteweg-de Vries/Kuramoto-Sivashinsky equation when subjected to classes of periodic perturbations. It is known that for each N is an element of N, such a T-periodic wave train is asymptotically stable to NT-periodic, i.e., subharmonic, perturbations, in the sense that initially nearby data will converge asymptotically to a small Galilean boost of the underlying wave, with exponential rates of decay. However, both the allowable size of initial perturbations and the exponential rates of decay depend on N and, in fact, tend to zero as N ->infinity, leading to a lack of uniformity in such subharmonic stability results. Our goal here is to build upon a recent methodology introduced by the authors in the reaction-diffusion setting and achieve a subharmonic stability result, which is uniform in N. This work is motivated by the dynamics of such wave trains when subjected to perturbations that are localized (i.e., integrable on the line).