Hajer Bahouri

Bio: Hajer Bahouri is an academic researcher from University of Paris. The author has contributed to research in topics: Sobolev space & Heisenberg group. The author has an hindex of 21, co-authored 69 publications receiving 3804 citations. Previous affiliations of Hajer Bahouri include Département de Mathématiques & Tunis University.

Fourier Analysis and Nonlinear Partial Differential Equations

Abstract: Preface.- 1. Basic analysis.- 2. Littlewood-Paley theory.- 3. Transport and transport-diffusion equations.- 4. Quasilinear symmetric systems.- 5. Incompressible Navier-Stokes system.- 6. Anisotropic viscosity.- 7. Euler system for perfect incompressible fluids.- 8. Strichartz estimates and applications to semilinear dispersive equations.- 9. Smoothing effect in quasilinear wave equations.- 10.- The compressible Navier-Stokes system.- References. - List of notations.- Index.

High frequency approximation of solutions to critical nonlinear wave equations

Abstract: This work is devoted to the description of bounded energy sequences of solutions to the equation (1) □ u + | u |4 = 0 in [inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="01i" /], up to remainder terms small in energy norm and in every Strichartz norm. The proof relies on scattering theory for (1) and on a structure theorem for bounded energy sequences of solutions to the linear wave equation. In particular, we infer the existence of an a priori estimate of Strichartz norms of solutions to (1) in terms of their energy.

Equations de transport relatives à des champs de vecteurs non-lipschitziens et mécanique des fluides

Abstract: Le but de cet article est l'etude des equations de transport relatives a des champs de vecteurs non-lipschitziens, mais seulement logarithmiquement lipschitziens. Ces champs possedent un flot dont la regularite holderienne est exponentiellement decroissante. On exhibe une solution de l'equation d'Euler bidimensionnelle presentant effectivement ce phenomene.

Equations d'ondes quasilineaires et estimations de strichartz

Abstract: Dans cet article, notre but est de resoudre des equations d'ondes quasilineaires pour des donnees initiales moins regulieres que ce qu'imposent les methodes d'energie. Ceci impose de demontrer des estimees de type Strichartz pour des operateurs d'ondes a coefficients seulement lipschitziens.

Decay estimates for the critical semilinear wave equation

Abstract: In this paper we prove that finite energy solutions (with added regularity) to the critical wave equation □u + u5 = 0 on R3 decay to zero in time. The proof is based on a global space-time estimate and dilation identity.

Table Of Integrals Series And Products

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Nonlinear dispersive equations : local and global analysis

Abstract: Ordinary differential equations Constant coefficient linear dispersive equations Semilinear dispersive equations The Korteweg de Vries equation Energy-critical semilinear dispersive equations Wave maps Tools from harmonic analysis Construction of ground states Bibliography.

Global well-posedness, scattering and blow-up for the energy-critical, focusing, non-linear Schrödinger equation in the radial case

Abstract: We prove, for the energy critical, focusing NLS, that for data whose energy is smaller than that of the standing wave, and whose homogeneous Sobolev norm H^1 is smaller than that of the standing wave and which is radial, we have global well-posedness and scattering in dimensions 3, 4 and 5. This is sharp since if the data is in the inhomogeneous Sobolev space H^1, of energy smaller than the standing wave but of larger homogeneous H^1 norm, we have blow-up in finite time. The result follows from a general method that we introduce into this type of critical problem. By concentration-compactness we produce a critical element, which modulo the symmetries of the equation is compact, has minimal energy among those which fail to have the conclusion of our theorem. In addition, we show that the dilation parameter in the symmetry, for this solution, can be taken strictly positive.We then establish a rigidity theorem that shows that no such compact, modulo symmetries, object can exist. It is only at this step that we use the radial hypothesis.The same analysis, in a simplified form, applies also to the defocusing case, giving a new proof of results of Bourgain and Tao.

A theory of regularity structures

Abstract: We introduce a new notion of “regularity structure” that provides an algebraic framework allowing to describe functions and/or distributions via a kind of “jet” or local Taylor expansion around each point. The main novel idea is to replace the classical polynomial model which is suitable for describing smooth functions by arbitrary models that are purpose-built for the problem at hand. In particular, this allows to describe the local behaviour not only of functions but also of large classes of distributions. We then build a calculus allowing to perform the various operations (multiplication, composition with smooth functions, integration against singular kernels) necessary to formulate fixed point equations for a very large class of semilinear PDEs driven by some very singular (typically random) input. This allows, for the first time, to give a mathematically rigorous meaning to many interesting stochastic PDEs arising in physics. The theory comes with convergence results that allow to interpret the solutions obtained in this way as limits of classical solutions to regularised problems, possibly modified by the addition of diverging counterterms. These counterterms arise naturally through the action of a “renormalisation group” which is defined canonically in terms of the regularity structure associated to the given class of PDEs. Our theory also allows to easily recover many existing results on singular stochastic PDEs (KPZ equation, stochastic quantisation equations, Burgers-type equations) and to understand them as particular instances of a unified framework. One surprising insight is that in all of these instances local solutions are actually “smooth” in the sense that they can be approximated locally to arbitrarily high degree as linear combinations of a fixed family of random functions/distributions that play the role of “polynomials” in the theory. As an example of a novel application, we solve the long-standing problem of building a natural Markov process that is symmetric with respect to the (finite volume) measure describing the $$\Phi ^4_3$$ Euclidean quantum field theory. It is natural to conjecture that the Markov process built in this way describes the Glauber dynamic of $$3$$ -dimensional ferromagnets near their critical temperature.