Opérateur de Fredholm

In mathematics, Fredholm operators are certain operators that arise in the Fredholm theory of integral equations. They are named in honour of Erik Ivar Fredholm. By definition, a Fredholm operator is a bounded linear operator T : X → Y between two Banach spaces with finite-dimensional kernel and finite-dimensional (algebraic) cokernel , and with closed range . The last condition is actually redundant. The index of a Fredholm operator is the integer or in other words, Intuitively, Fredholm operators are those operators that are invertible "if finite-dimensional effects are ignored." The formally correct statement follows. A bounded operator T : X → Y between Banach spaces X and Y is Fredholm if and only if it is invertible modulo compact operators, i.e., if there exists a bounded linear operator such that are compact operators on X and Y respectively. If a Fredholm operator is modified slightly, it stays Fredholm and its index remains the same. Formally: The set of Fredholm operators from X to Y is open in the Banach space L(X, Y) of bounded linear operators, equipped with the operator norm, and the index is locally constant. More precisely, if T0 is Fredholm from X to Y, there exists ε > 0 such that every T in L(X, Y) with ||T − T0|| < ε is Fredholm, with the same index as that of T0. When T is Fredholm from X to Y and U Fredholm from Y to Z, then the composition is Fredholm from X to Z and When T is Fredholm, the transpose (or adjoint) operator T ′ is Fredholm from Y ′ to X ′, and ind(T ′) = −ind(T). When X and Y are Hilbert spaces, the same conclusion holds for the Hermitian adjoint T∗. When T is Fredholm and K a compact operator, then T + K is Fredholm. The index of T remains unchanged under such a compact perturbations of T. This follows from the fact that the index i(s) of T + s K is an integer defined for every s in [0, 1], and i(s) is locally constant, hence i(1) = i(0). Invariance by perturbation is true for larger classes than the class of compact operators.

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Opérateur compact

En mathématiques, et plus précisément en analyse fonctionnelle, un opérateur compact est une application continue entre deux espaces vectoriels topologiques X et Y envoyant les parties bornées de X sur les parties relativement compactes de Y. Les applications linéaires compactes généralisent les applications linéaires continues de rang fini. La théorie est particulièrement intéressante pour les espaces vectoriels normés ou les espaces de Banach. En particulier, dans un espace de Banach, l'ensemble des opérateurs compacts est fermé pour la topologie forte.

Opérateur de Fredholm

En mathématiques, l'opérateur de Fredholm est un concept d'analyse fonctionnelle qui porte le nom du mathématicien suédois Ivar Fredholm (1866-1927). Il s'agit d'un opérateur borné L entre deux espaces de Banach X et Y ayant un noyau de dimension finie et une image de codimension finie. On peut alors définir l'indice de l'opérateur comme Sous ces hypothèses, l'espace image de L est fermé (il admet même un supplémentaire topologique).

Espace de Hilbert

vignette|Une photographie de David Hilbert (1862 - 1943) qui a donné son nom aux espaces dont il est question dans cet article. En mathématiques, un espace de Hilbert est un espace vectoriel réel (resp. complexe) muni d'un produit scalaire euclidien (resp. hermitien), qui permet de mesurer des longueurs et des angles et de définir une orthogonalité. De plus, un espace de Hilbert est complet, ce qui permet d'y appliquer des techniques d'analyse. Ces espaces doivent leur nom au mathématicien allemand David Hilbert.

Exponential decay and Fredholm properties in second-order quasilinear elliptic systems

Charles Stuart, Hicham Georges Gebran

We consider second-order quasilinear elliptic systems on un-bounded domains in the setting of Sobolev spaces. We complete our earlier work on the Fredholm and properness properties of the associated differential operators by giving verifiable conditions for the linearization to be Fredholm of index zero. This opens the way to using the degree for C-1-Fredholm maps of index zero as a tool in the study of such quasilinear systems. Our work also enables us to check the Fredholm assumption which plays an important role in Rabies approach to proving exponential decay to zero at infinity of solutions. (C) 2010 Elsevier Inc. All rights reserved.

2010

Global Continuation for Quasilinear Elliptic Systems on R-N and the Equations of Elastostatics

Charles Stuart, Hicham Georges Gebran

We consider quasilinear systems of second order elliptic equations on R-N. Using a continuation theorem based on the topological degree for C-1-Fredholm maps, we derive global properties of a maximal connected set of solutions which decay exponentially to zero at infinity. These results are used to treat a problem concerning the equilibrium of an elastic body occupying the whole space and subjected to a one parameter family of localized external forces.

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Global continuation for ODE systems over the half-line and study of von Kármán's swirling flow problem

Gilles Evéquoz

This work is concerned with the global continuation for solutions (λ,u,ξ) ∈ R × C1{0}([0,∞), RN) × Rk of the following system of ordinary differential equations: where F: [0,∞) × RN × U × J → RN and φ: U × J → X1, for some open sets J ⊂ R and U ⊂ Rk, and where RN = X1 ⊕ X2 is a given decomposition, with associated projection P: RN → X1. This problem gives rise to a nonlinear operator whose zeros correspond exactly to the solutions of the original problem. We give conditions on F and φ ensuring that this operator has the Fredholm property and is proper on the closed bounded subsets of R × C1{0}([0,∞), RN) × Rk. These conditions generalize to a parameter dependent situation some recent results obtained by Morris [24]. Under these assumptions, we study the global behavior of a particular connected set of solutions using a degree theory available for such operators and obtain global continuation theorems. In the second part of this work, we use our general results to prove the existence of solutions for the so-called swirling flow problem in fluid dynamics, which can be written as a system of two ordinary differential equations on the half-line together with boundary conditions. Having obtained a priori bounds on possible solutions to this problem, we are able to recover the existence result obtained by Mcleod [22] using ad hoc arguments. In the last part, we present some numerical computations and give pictures of solutions to this problem.

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