The scattering matrix is defined on a perturbed stratified medium. For a class of perturbations, its main part at fixed energy is a Fourier integral operator on the sphere at infinity. Proving this is facilitated by developing a refined limiting absorption principle. The symbol of the scattering matrix determines the asymptotics of a large class of perturbations.

We extend our recent results on propagation of semiclassical resolvent estimates through trapped sets when a priori polynomial resolvent bounds hold. Previously we obtained non-trapping estimates in trapping situations when the resolvent was sandwiched between cutoffs $\chi$ microlocally supported away from the trapping: $\parallel \chi R_h(E+i0)\chi \parallel =𝒪\left(h^{-1}\right)$, a microlocal version of a result of Burq and Cardoso-Vodev. We now allow one of the two cutoffs, $\tilde{\chi }$, to be supported at the trapped set, giving $\parallel \chi R_h(E+i0)\tilde{\chi }\parallel =𝒪\left(\sqrt{a\left(h\right)}{h}^{-1}\right)$ when the a priori bound is $\parallel \tilde{\chi }{R}_h(E+i0)\tilde{\chi }\parallel =𝒪\left(a\left(h\right)h^{-1}\right)$.

Dans ce travail, on s’intéresse à l’existence globale de solutions classiques et au sens de Shatah-Struwe de l’équation des ondes critique à coefficients variables en dimension $d$ d’espace$${\square }_{A}u+{\left|u\right|}^{4/(d-2)}u={\partial }_t^{2}u-\mathrm{div}(A\left(x\right)·{\nabla }_{x}u)+{\left|u\right|}^{4/(d-2)}u=0,{ℝ}_t\times {ℝ}_x^d,$$où $A$ est une fonction régulière à valeurs dans les matrices $d\times d$ définies positives, valant l’identité en dehors d’un compact fixe.

This article is a proceedings version of the ongoing work [1], and has been the object of a talk of the second author during the Journées “Équations aux Dérivées Partielles” (Biarritz, 2012).We address the decay rates of the energy of the damped wave equation when the damping coefficient $b$ does not satisfy the Geometric Control Condition (GCC). First, we give a link with the controllability of the associated Schrödinger equation. We prove that the observability of the Schrödinger group implies that...

We consider a family of conforming finite element schemes with piecewise polynomial space of degree $k$ in space for solving the wave equation, as a model for second order hyperbolic equations. The discretization in time is performed using the Newmark method. A new a priori estimate is proved. Thanks to this new a priori estimate, it is proved that the convergence order of the error is $h^k+{\tau }^2$ in the discrete norms of ${ℒ}^{\infty }(0,T;{\mathscr{H}}^1\left(\Omega \right))$ and ${𝒲}^{1,\infty }(0,T;{ℒ}^2\left(\Omega \right))$, where $h$ and $\tau$ are the mesh size of the spatial and temporal discretization, respectively....