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We enlarge the class of Beltrami equations by developping a stability theory for the sheaf of solutions of an overdetermined elliptic system of first order homogeneous partial differential equations with constant coefficients in the Euclidean space.

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La problématique centrale de cette thèse est l'élaboration d'un opérateur de Laplace--Beltrami discret sur les surfaces digitales. Ces surfaces proviennent de la théorie de la géométrie discrète, c’est-à-dire la géométrie qui s'intéresse à des sous-ensembles des entiers relatifs. Nous nous plaçons ici dans un cadre théorique où les surfaces digitales sont le résultat d'une approximation, ou processus de discrétisation, d'une surface continue sous-jacente. Cette méthode permet à la fois de prouver des théorèmes de convergence des quantités discrètes vers les quantités continues, mais aussi, par des analyses numériques, de confirmer expérimentalement ces résultats. Pour la discrétisation de l’opérateur, nous faisons face à deux problèmes : d'un côté, notre surface n'est qu'une approximation de la surface continue sous-jacente, et de l'autre côté, l'estimation triviale de quantités géométriques sur la surface digitale ne nous apporte pas en général une bonne estimation de cette quantité. Nous possédons déjà des réponses au second problème : ces dernières années, de nombreux articles se sont attachés à développer des méthodes pour approximer certaines quantités géométriques sur les surfaces digitales (comme par exemple les normales ou bien la courbure), méthodes que nous décrirons dans cette thèse. Ces nouvelles techniques d'approximation nous permettent d'injecter des informations de mesure sur les éléments de notre surface. Nous utilisons donc l'estimation de normales pour répondre au premier problème, qui nous permet en fait d'approximer de façon précise le plan tangent en un point de la surface et, via une méthode d'intégration, palier à des problèmes topologiques liées à la surface discrète. Nous présentons un résultat théorique de convergence du nouvel opérateur discrétisé, puis nous illustrons ensuite ses propriétés à l’aide d’une analyse numérique de l’opérateur. Nous effectuons une comparaison détaillée du nouvel opérateur par rapport à ceux de la littérature adaptés sur les surfaces digitales, ce qui nous permet, au moins pour la convergence, de montrer que seul notre opérateur possède cette propriété. Nous illustrons également l’opérateur via quelques unes de ces applications comme sa décomposition spectrale ou bien encore le flot de courbure moyenne / The central issue of this thesis is the development of a discrete Laplace--Beltrami operator on digital surfaces. These surfaces come from the theory of discrete geometry, i.e. geometry that focuses on subsets of relative integers. We place ourselves here in a theoretical framework where digital surfaces are the result of an approximation, or discretization process, of an underlying smooth surface. This method makes it possible both to prove theorems of convergence of discrete quantities towards continuous quantities, but also, through numerical analyses, to experimentally confirm these results. For the discretization of the operator, we face two problems: on the one hand, our surface is only an approximation of the underlying continuous surface, and on the other hand, the trivial estimation of geometric quantities on the digital surface does not generally give us a good estimate of this quantity. We already have answers to the second problem: in recent years, many articles have focused on developing methods to approximate certain geometric quantities on digital surfaces (such as normals or curvature), methods that we will describe in this thesis. These new approximation techniques allow us to inject measurement information into the elements of our surface. We therefore use the estimation of normals to answer the first problem, which in fact allows us to accurately approximate the tangent plane at a point on the surface and, through an integration method, to overcome topological problems related to the discrete surface. We present a theoretical convergence result of the discretized new operator, then we illustrate its properties using a numerical analysis of it. We carry out a detailed comparison of the new operator with those in the literature adapted on digital surfaces, which allows, at least for convergence, to show that only our operator has this property. We also illustrate the operator via some of these applications such as its spectral decomposition or the mean curvature flow

Neste trabalho obtemos taxas de decaimento para autovalores e valores singulares de operadores integrais gerados por núcleos de quadrado integrável sobre a esfera unitária em \'R POT. m+1\', m 2, sob hipóteses sobre ambos, certas derivadas do núcleo e o operador integral gerado por tais derivadas. Este tipo de problema é comum na literatura, mas as hipóteses geralmente são definidas via diferenciação usual em \'R POT m+1\'. Aqui, as hipóteses são todas definidas via derivada de Laplace-Beltrami, um conceito genuinamente esférico investigado primeiramente por W. Rudin no começo dos anos 50. As taxas de decaimento apresentadas são ótimas e dependem da dimensão m e da ordem de diferenciabilidade usada para definir as condições de suavidade / In this work we obtain decay rates for singular values and eigenvalues of integral operators generated by square integrable kernels on the unit sphere in \'R m+1\', m 2, under assumptions on both, certain derivatives of the kernel and the integral operators generated by such derivatives. This type of problem is common in the literature but the assumptions are usually defined via standard differentiation in \'R POT. m+1\'. Here, the assumptions are all defined via the Laplace-Beltrami derivative, a concept first investigated by W. Rudin in the early fifties and genuinely spherical in nature. The rates we present are optimal and depend on both, the differentiability order used to define the smoothness conditions and the dimension m

Neste trabalho estudamos diferentes noções de diferenciabilidade para funções definidas na esfera unitária S^n-1 de R^n, n>=2. Em relação à derivada usual, encontramos condições necessárias e/ou suficientes para que uma função seja diferenciável até uma ordem fixada. Para as outras duas, a derivada forte de Laplace-Beltrami e a derivada fraca, apresentamos algumas propriedades básicas e procuramos condições que garantam a equivalência destas com a diferenciabilidade usual. / In this work we study different notions of differentiability for functions defined on the unit sphere S^n-1 of R^n, n>=2. With respect to the usual derivative, we find necessary and/or sufficient conditions in order that a function be differentiable up to a fixed order. As for the other two, the strong Laplace-Beltrami derivative and the weak derivative, we present some basic properties about them and search for conditions that guarantee the equivalence of them with the previous one.

Neste trabalho estudamos diferentes noções de diferenciabilidade para funções definidas na esfera unitária S^n-1 de R^n, n>=2. Em relação à derivada usual, encontramos condições necessárias e/ou suficientes para que uma função seja diferenciável até uma ordem fixada. Para as outras duas, a derivada forte de Laplace-Beltrami e a derivada fraca, apresentamos algumas propriedades básicas e procuramos condições que garantam a equivalência destas com a diferenciabilidade usual. / In this work we study different notions of differentiability for functions defined on the unit sphere S^n-1 of R^n, n>=2. With respect to the usual derivative, we find necessary and/or sufficient conditions in order that a function be differentiable up to a fixed order. As for the other two, the strong Laplace-Beltrami derivative and the weak derivative, we present some basic properties about them and search for conditions that guarantee the equivalence of them with the previous one.

Neste trabalho obtemos taxas de decaimento para autovalores e valores singulares de operadores integrais gerados por núcleos de quadrado integrável sobre a esfera unitária em \'R POT. m+1\', m 2, sob hipóteses sobre ambos, certas derivadas do núcleo e o operador integral gerado por tais derivadas. Este tipo de problema é comum na literatura, mas as hipóteses geralmente são definidas via diferenciação usual em \'R POT m+1\'. Aqui, as hipóteses são todas definidas via derivada de Laplace-Beltrami, um conceito genuinamente esférico investigado primeiramente por W. Rudin no começo dos anos 50. As taxas de decaimento apresentadas são ótimas e dependem da dimensão m e da ordem de diferenciabilidade usada para definir as condições de suavidade / In this work we obtain decay rates for singular values and eigenvalues of integral operators generated by square integrable kernels on the unit sphere in \'R m+1\', m 2, under assumptions on both, certain derivatives of the kernel and the integral operators generated by such derivatives. This type of problem is common in the literature but the assumptions are usually defined via standard differentiation in \'R POT. m+1\'. Here, the assumptions are all defined via the Laplace-Beltrami derivative, a concept first investigated by W. Rudin in the early fifties and genuinely spherical in nature. The rates we present are optimal and depend on both, the differentiability order used to define the smoothness conditions and the dimension m

In this thesis we study the asymptotic behavior of bi-Lipschitz diffeomorphic weighted Riemannian manifolds with techniques from the theory of homogenization. To do so we re-interpret the problem as different induced metrics on one reference manifold. Our analysis is twofold. On the one hand we consider second-order uniformly elliptic operators on weighted Riemannian manifolds. They naturally emerge when studying spectral properties of the Laplace-Beltrami operator on families of manifolds with rapidly oscillating metrics. We appeal to the notion of H-convergence introduced by Murat and Tartar. In our first main result we establish an H-compactness result that applies to elliptic operators with measurable, uniformly elliptic coefficients on weighted Riemannian manifolds. We further discuss the special case of locally periodic coefficients and study the asymptotic spectral behavior of Euclidean submanifolds with rapidly oscillating geometry. On the other hand we study integral functionals featuring non-convex integrands with non-standard growth on the Euclidean space in a stochastic framework. Our second main result is a Γ-convergence statement under certain assumptions on the statistics of their integrands. Such functionals provide a tool to study the Dirichlet energy on non-uniformly bi-Lipschitz diffeomorphic manifolds. We show Mosco-convergence of the Dirichlet energy and deduce conditions for the spectral behavior of weighted Riemannian manifolds with locally oscillating random structure, especially in the case of Euclidean submanifolds.:Introduction Outline Notation I. Preliminaries 1. Convergence of Riemannian Manifolds 1.1. Hausdorff-Convergence 1.2. Gromov-Hausdorff-Convergence 1.3. Spectral Convergence 1.4. Mosco-Convergence 2. Homogenization 2.1. Periodic Homogenization 2.2. Stochastic Homogenization II. Uniformly bi-Lipschitz Diffeomorphic Manifolds 3. Uniformly Elliptic Operators on a Riemannian Manifold 3.1. Setting 3.2. Main Results 3.3. Strategy of the Proof and Auxiliary Results 3.4. Identi cation of the Limit via Local Coordinate Charts 3.5. Examples 3.6. Proofs 4. Application to Uniformly bi-Lipschitz Diffeomorphic Manifolds 4.1. Setting and Results 4.2. Examples 4.3. Proofs III. Rapidly Oscillating Random Manifolds 5. Integral Functionals with Non-Uniformal Growth 5.1. Setting 5.2. Main Results 5.3. Strategy of the Proof and Auxiliary Results 5.4. Proofs 6. Application to Rapidly Oscillating Riemannian Manifolds 6.1. Setting and Results 6.2. Examples 6.3. Proofs Summary and Discussion Bibliography List of Figures

We introduce the concept of Toeplitz operator associated with the Laplace-Beltrami operator on a compact Riemannian manifold with boundary. We characterise those Toeplitz operators which are Fredholm, thus initiating the index theory.