Stability of periodic waves for the fractional KdV and NLS equations

Abstract: We consider the focusing fractional periodic Korteweg-deVries (fKdV) and fractional periodic nonlinear Schr\"odinger equations (fNLS) equations, with $L^2$ sub-critical dispersion. In particular, this covers the case of the periodic KdV and Benjamin-Ono models. We construct two parameter family of bell-shaped traveling waves for KdV (standing waves for NLS), which are constrained minimizers of the Hamiltonian. We show in particular that for each $\lambda>0$, there is a traveling wave solution to fKdV and fNLS $\varphi: |\varphi|_{L^2[-T,T]}^2=\lambda$, which is non-degenerate and spectrally stable, as well as orbitally stable. This is done completely rigorously, without any {\it a priori} assumptions on the smoothness of the waves or the Lagrange multipliers.