A geometric proof of the Quasi-linearity of the water-waves system

Abstract: In the first part of this paper we prove that the flow associated to the Burgers equation with a non local term of the form $\partial_x |D|^{\alpha-1} u$ fails to be uniformly continuous from bounded sets of $H^s({\mathbb D})$ to $C^0([0,T],H^s({\mathbb D}))$ for $T>0$, $s>\frac{1}{2}+2$, $0\leq \alpha <2$, ${\mathbb D}={\mathbb R} \ \text{or} \ {\mathbb T} $. Furthermore we show that the flow cannot be $C^1$ from bounded sets of $H^s({\mathbb D})$ to $C^0([0,T],H^{s-1+(\alpha-1)^+ +\epsilon}({\mathbb D}))$ for $\epsilon>0$. We generalize this result to a large class of nonlinear transport-dispersive equations in any dimension, that in particular contains the Whitham equation and the paralinearization of the water waves system with and without surface tension. The current result is optimal in the sense that for $\alpha=2$ and ${\mathbb D}={\mathbb T}$ the flow associated to the Benjamin-Ono equation is Lipschitz on function with $0$ mean value $H^s_0$. In the second part of this paper we apply this method to deduce the quasi-linearity of the water waves system, which is the main result of this paper.