Fonction propre

Résumé

En mathématiques Théorie spectrale En théorie spectrale, une fonction propre f d'un opérateur linéaire \mathcal A sur un espace fonctionnel est un vecteur propre de l'opérateur linéaire. En d’autres termes, une fonction propre d'un opérateur linéaire, \mathcal A, défini sur un certain espace de fonction, est toute fonction f non identiquement nulle sur cet espace qui, lorsqu’elle se voit appliquer cet opérateur en ressort exactement pareille à elle-même, à un facteur d'échelle multiplicatif près. Cette fonction satisfait donc : :\mathcal A f = \lambda f pour un scalaire λ, la valeur propre associée à f. L'existence de vecteurs propres est typiquement de grand secours pour analyser \mathcal A. Par exemple, pour tout réel \ \alpha, f_\alpha : \R \to \R,,x \mapsto e^{\alpha x} est une fonction propre pour l'opérateur différentiel :\mathcal A = \frac{\mathrm d^2}{\mathrm dx^2} -

Source officielle

https://fr.wikipedia.org/wiki/Fonction_propre

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Finite Element Method on Riemann Surfaces and Applications to the Laplacian Spectrum

Marc Maintrot

The objective of this PhD thesis is the approximate computation of the solutions of the Spectral Problem associated with the Laplace operator on a compact Riemann surface without boundaries. A Riemann surface can be seen as a gluing of portions of the Hyperbolic Plane made with suitable conditions to obtain a 2 dimensional manifold. The solutions of the Spectral Problem associated with the Laplace operator are to be understood as the eigenfunctions defined on the surface and their corresponding eigenvalues. This work is separated into two parts: the first part describes the method used to approximate the eigenvalues and eigenfunctions, the second focuses on the design of a program to compute these approximations. The approximation method is inspired by the Finite Element Method (FEM), in that it relies on the variational expression of the Spectral Problem and the definition of a finite subspace of functions in which the approximated eigenvalues and eigenfunctions are computed. However, it differs from the FEM in that it removes the euclidian basis of the FEM and is invariant under the isometries of the Hyperbolic Plane. To ful fill this objective, we begin by geodesically triangulating the surface as regularly as possible. This hyperbolic triangulation allows us to de ne the finite subspace of functions by using the concept of barycentric coordinates associated with each vertex of the triangulation (idea introduced by Whitney and taken up by Dodziuk). We then prove that the approximated solutions convergence to the exact ones when the diameter of the triangulation decreases, as well as the order of convergence. The program is a practical application of the theoretical work and allows the computation of the approximated eigenfunctions and eigenvalues.

EPFL2012

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Cakoni and Nguyen recently proposed very general conditions on the coefficients of Maxwell equations for which they established the discreten ess of the set of eigenvalues of the transmission problem and studied their locations. In this paper, we establish the completeness of the generalized eigenfunctions and derive an optimal upper bound for the counting function under these conditions, assuming additionally that the coefficients are twice continuously differentiable. The approach is based on the spectral theory of Hilbert-Schmidt operators.

2021

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Eigenfunction expansions of ultradifferentiable functions and ultradistributions

Aparajita Dasgupta

In this paper we give a global characterisation of classes of ultradi_erentiable functions and corresponding ultradistributions on a compact manifold X. The characterisation is given in terms of the eigenfunction expansion of an elliptic operator on X. This extends the result for analytic functions on compact manifold by Seeley [See69], and the characterisation of Gevrey functions and Gevrey ultradistributions on compact Lie groups and homogeneous spaces by the authors.

2016

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