Uniform asymptotic formulae for Green's kernels in regularly and singularly perturbed domains

Asymptotic formulae for Green's kernels $G_{\epsilon }(\mathbf{x},\mathbf{y})$ of various boundary value problems for the Laplace operator are obtained in regularly perturbed domains and certain domains with small singular perturbations of the boundary, as $\epsilon \to 0$. The main new feature of these asymptotic formulae is their uniformity with respect to the independent variables x and y. To cite this article: V. Maz'ya, A. Movchan, C. R. Acad. Sci. Paris, Ser. I 343 (2006).     

Monotone approximations of Green's functions

We study the approximations of the Green's function $\mathbb{G}$ in a domain Ω obtained from an approximation of the Dirac mass ${\delta }_0$. We prove that under some conditions, these approximations converge monotonically to $\mathbb{G}$, a rather surprising result. To cite this article: E. Chasseigne, R. Ferreira, C. R. Acad. Sci. Paris, Ser. I 339 (2004).     

Étude d'un modèle thermo-chimique de formation d'un matériau composite

We consider a composite material constituted of carbon or glass fibres included in a resin which becomes solid when it is heated up (reaction of reticulation). The mathematical modelling of the cure process is given by a kinetic equation describing the evolution of the reaction of reticulation coupled with the heat equation. The geometry of the composite material is periodic, with a small period $?>0$. First we prove the existence and uniqueness of a solution by using Schauder's fixed point theorem. Then, by using an asymptotic expansion, we derive the homogenized problem which describes the macroscopic behaviour of the material. We prove the convergence of the solution of the problem to the solution of the homogenized problem when ? tends to zero and we obtain an error estimate in a case of weak non-linearity. Finally we solve numerically the homogenized problem. To cite this article: S. Meliani et al., C. R. Acad. Sci. Paris, Ser. I 339 (2004).     

Boundary value problems with nonhomogeneous Neumann conditions on a fractal boundary

This note deals with some boundary value problems in a self-similar ramified domain of ${\mathbb{R}}^2$, with a fractal boundary. The partial differential equation is Laplace's equation, and there are nonhomogeneous generalized Neumann boundary conditions on the fractal boundary. We propose a multiscale strategy for approximating the restriction of the solutions to simple subdomains. This strategy is based on transparent boundary conditions and on a wavelet expansion of the Neumann datum. To cite this article: Y. Achdou, N. Tchou, C. R. Acad. Sci. Paris, Ser. I 342 (2006).     

Beltrami's solutions of general equilibrium equations in continuum mechanics

M. Gurtin has proved that the Beltrami representation, $\mathbf{S}=\mathrm{rot}\mathrm{rot}\mathbf{A}$, of a smooth, divergence-free stress tensor in a smooth domain, is verified if and only if S is self-equilibrated. Here, Gurtin's conditions are extended to the case of a bounded domain with a Lipschitz-continuous boundary, for a tensor field $\mathbf{S}\in L^2(\Omega ;M_{\mathrm{sym}}^3)$. We apply this result to obtain an extension of the Saint Venant's equations of compatibility to non necessarily simply-connected domains. To cite this article: G. Geymonat, F. Krasucki, C. R. Acad. Sci. Paris, Ser. I 342 (2006).     

The asymptotically sharp Korn interpolation and second inequalities for shells

We consider shells in three-dimensional Euclidean space that have bounded principal curvatures. We prove Korn's interpolation (or the so-called first and a half¹) and the second inequalities on that kind of shells for $\mathit{u}\in W^{1,2}$ vector fields, imposing no boundary or normalization conditions on u. The constants in the estimates are optimal in terms of the asymptotics in the shell thickness h, having the scalings h or $O(1)$. The Korn interpolation inequality reduces the problem of deriving any linear Korn type estimate for shells to simply proving a Poincaré-type estimate with the symmetrized gradient on the right-hand side. In particular, this applies to linear geometric rigidity estimates for shells, i.e. Korn's fist inequality without boundary conditions.     

Solutions globales d'énergie infinie de l'équation de Navier–Stokes 2D

We study in this Note the solutions of the 2D Navier–Stokes equations with initial data in ?BMO. For $u{|}_{t=0}$ in the closure of the Schwartz class, we obtain the existence and uniqueness of a global solution, and besides an estimate on its norm in ?BMO. To cite this article: P. Germain, C. R. Acad. Sci. Paris, Ser. I 340 (2005).     

The Helmholtz equation with impedance in a half-plane

This Note gives answers to the uniqueness and existence questions for solutions of the Helmholtz equation in an half-plane with an impedance or mixed boundary condition. We deal with unbounded domains which boundaries are unbounded too. The radiation conditions are different from the ones that we found in an usual exterior problem due to the appearance of surface waves. We first compute and study the half-plane Green's function to see how the solutions behave at infinity, and second obtain integral representation for these solutions. To cite this article: M. Duran et al., C. R. Acad. Sci. Paris, Ser. I 340 (2005).     

Sharp Hardy–Sobolev inequalities

Let Ω be a smooth bounded domain in ${\mathbb{R}}^N$, $N?3$. We show that Hardy's inequality involving the distance to the boundary, with best constant ($\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$4$}\right.$), may still be improved by adding a multiple of the critical Sobolev norm. To cite this article: S. Filippas et al., C. R. Acad. Sci. Paris, Ser. I 339 (2004).     

Qualitative properties of nodal solutions of semilinear elliptic equations in radially symmetric domains

We study the qualitative properties of sign changing solutions of the Dirichlet problem $\mathrm{\Delta }u+f(u)=0$ in Ω, $u=0$ on ?Ω, where Ω is a ball or an annulus and f is a $C^1$ function with $f(0)?0$. We prove that any radial sign changing solution has a Morse index bigger or equal to $N+1$ and give sufficient conditions for the nodal surface of a solution to intersect the boundary. In particular, we prove that any least energy nodal solution is non radial and its nodal surface touches the boundary. To cite this article: A. Aftalion, F. Pacella, C. R. Acad. Sci. Paris, Ser. I 339 (2004).     

Sur un problème de shallow water bicouche avec conditions aux limites de Dirichlet

We present in this Note an existence result for a bi-layer shallow water problem, with Dirichlet boundary conditions. The main difficulty arises from the terms coupling the two layers. To handle these, we must couple the natural energy estimate with an estimation of the thickness in $L^2(Q)$. To obtain this $L^2$-estimate, we introduce the Stokes operator and we use results for Hardy spaces to treat the nonlinear terms. To cite this article: F. Flori et al., C. R. Acad. Sci. Paris, Ser. I 340 (2005).     

Borne sur l'advection et la hauteur d'eau du problème de shallow-water avec conditions aux limites de Dirichlet

Using some properties of the Hardy spaces, we give a regularity result on the advection term of the shallow-water equations and we show a $L^2$ bound holding up to the boundary on the water height with Dirichlet boundary conditions. To cite this article: F. Flori, P. Orenga, C. R. Acad. Sci. Paris, Ser. I 340 (2005).     

Inégalités d'oracle pour l'estimation d'une densité de probabilité

We study the problem of the nonparametric estimation of a probability density in $L_2(\mathbb{R})$. Expressing the mean integrated squared error in the Fourier domain, we show that it is close to the mean squared error in the Gaussian sequence model. Then, applying a modified version of Stein's blockwise method, we obtain a linear monotone oracle inequality and a kernel oracle inequality. As a consequence, the proposed estimator is sharp minimax adaptive (i.e. up to a constant) on a scale of Sobolev classes of densities. To cite this article: Ph. Rigollet, C. R. Acad. Sci. Paris, Ser. I 340 (2005).     

Une approche polynomiale du théorème de Faltings

We present here an elementary proof of a quantitatively improved version of the Mordell's Conjecture which is now Faltings's Theorem. For the count of the ‘large points’ of $C(K)$ (see below for the notations) we use Vojta's method which was simplified by Bombieri and then by T. de Diego, G. Rémond. To count the points of small heights of $C(K)$, we use a result of S. David and P. Philippon, allowing to us estimate the number of points of small height of the bigger set $C(\overline{K})\cap J(K)$. To cite this article: B. Farhi, C. R. Acad. Sci. Paris, Ser. I 340 (2005).     

Superrigidity for irreducible lattices and geometric splitting

We propose general superrigidity results for actions of irreducible lattices on $\mathrm{CAT}(0)$ spaces. In particular, we obtain a new and self-contained proof of Margulis' superrigidity theorem for uniform irreducible lattices in non-simple groups. However, the statements hold for lattices in products of arbitrary groups; likewise, the geometric representations need not be linear. The proof uses notably a new splitting theorem which can be viewed as an infinite-dimensional and singular generalization of the Lawson–Yau/Gromoll–Wolf theorem. To cite this article: N. Monod, C. R. Acad. Sci. Paris, Ser. I 340 (2005).