Smoothing estimates for non-dispersive equations

This paper describes an approach to global smoothing problems for non-dispersive equations based on ideas of comparison principle and canonical transformation established in authors' previous paper, where dispersive equations were treated. For operators $a(D_x)$ of order $m$ satisfying the dispersiveness condition $\nabla a(\xi)\neq0$ for $\xi\not=0$, the global smoothing estimate $$ \|\langle x\rangle^{-s}|D_x|^{(m-1)/2}e^{ita(D_x)} \varphi(x)\|_{L^2(\mathbb R_t\times\mathbb R^n_x)} \leq C\|\varphi\|_{L^2(\mathbb R^n_x)} \quad {\rm(}s>1/2{\rm)} $$ is well-known, while it is also known to fail for non-dispersive operators. For the case when the dispersiveness breaks, we suggest the estimate in the form $$ \|{\langle{x}\rangle^{-s}|\nabla a(D_x)|^{1/2} e^{it a(D_x)}\varphi(x)}\|_{L^2({\mathbb R_t\times\mathbb R^n_x})} \leq C\|{\varphi}\|_{L^2({\mathbb R^n_x})}\quad{\rm(}s>1/2{\rm)} $$ which is equivalent to the usual estimate in the dispersive case and is also invariant under canonical transformations for the operator $a(D_x)$. We show that this estimate and its variants do continue to hold for a variety of non-dispersive operators $a(D_x)$, where $\nabla a(\xi)$ may become zero on some set. Moreover, other types of such estimates, and the case of time-dependent equations are also discussed.