The main object of study is the theory of Schrödinger operators with meromorphic potentials, having trivial monodromy in the complex domain. In the first part we study the spectral properties of a class of such operators related to the classical Whittaker-Hill equation (-d^2/dx^2+Acos2x+Bcos4x)Ψ=λΨ. The equation, for special choices of A and B, is known to have the remarkable property that half of the gaps eventually become closed (semifinite-gap operator). Using the Darboux transformation we construct new trigonometric examples of semifinite-gap operators with real, smooth potentials. A similar technique applied to the Lamé operator gives smooth, real, finite-gap potentials in terms of classical Jacobi elliptic functions. In the second part we study the singular locus of monodromy-free potentials in the complex domain. A particular case is given by the zeros of Wronskians of Hermite polynomials, which are studied in detail. We introduce a class of partitions (doubled partitions) for which we observe a direct qualitative relationship between the pattern of zeros and the shape of the corresponding Young diagram. For the Wronskians W(H_n,H_{n+k}) we give an asymptotic formula for the curve on which zeros lie as n → ∞. We also give some empirical formulas for asymptotic behaviour of zeros of Wronskians of 3 and 4 Hermite polynomials. In the last chapter we apply the theory of monodromy-free operators to produce new vortex equilibria in the periodic case and in the presence of background flow.

We derive the ground-state eigenvalues and eigenvectors for a simplified version of the 1-D QED single electron-photon model that Glasgow et al recently developed [2]. This model still allows for meaningful interaction between electrons and photons while keeping only the minimum needed to do so. We investigate the interesting spectral properties of this model. We determine that the eigenvectors are orthogonal as one would expect and normalize them.

Spectral properties of integral operators on spaces of analytic functions on the unit disk of the complex plane have been studied since 1918. In this dissertation we determine spectral pictures and resolvent estimates for Ces`aro-like operators on the weighted Bergman spaces and show in particular that some of these operators are subdecomposable. Moreover, in a special case, we show that some of these operators are subnormal, some are normaloid, and some are subscalar. We also determine the spectrum and essential spectrum as well as resolvent estimates for a class of integral operators acting on Banach spaces of analytic functions on the unit disk, including the classical Hardy and weighted Bergman spaces, analytic Besov spaces as well as certain Dirichlet spaces and generalized Bloch spaces. Our results unify and extend recent work by Aleman and Persson, [4], Ballamoole, Miller and Miller, [6], and Albrecht and Miller, [3]. In [3], another class of integral operators were investigated in the setting of the analytic Besov spaces and the little Bloch space where the spectra, essential spectra together with one sided analytic resolvents in the Fredholm regions of these operators were obtained along with an explicit strongly decomposable operator extending one of these operator and simultaneously lifting the other. In this disseration, we extend this spectral analysis to nonseparable generalized Bloch spaces using a modification of a construction due to Aleman and Persson, [4].

[pt] Este trabalho trata o Método Híbrido dos Elementos de Contorno com vista à análise de problemas que envolvam simetria ou antissimetria. Nestes casos, apenas uma parte da estrutura, que pode ser a metade, um quarto ou um oitavo, deve ser discretizada e capaz de representar o todo. Os métodos de contorno apresentam a vantagem, quando comparados com os de domínio, de não ser necessário nenhum tipo de discretização ao longo dos eixos ou planos de simetria, sem a introdução de mais aproximações, visto que apenas o contorno é discretizado. Embora estas simplificações venham a restringir alguns deslocamentos de corpo rígido (para problemas de elasticidade), no Método dos Elementos de Contorno convencional (colocação ou Galerkin) a ausência de tais deslocamentos não acarreta alterações na sistemática do método. Nos Métodos Híbridos de Elementos de Contorno, por outro lado, os deslocamentos de corpo rígido são necessários direta ou indiretamente para a aplicação de condições de ortogonalidade e avaliação das propriedades espectrais que são essenciais na obtenção da diagonal principal de certas matrizes inerentes ao método, tais como de flexibilidade, de deslocamentos e de tensões. Esta necessidade de avaliação é uma característica de suma importância do método e, quando não houver possibilidade de fazê-la, deve-se procurar uma forma substituta conceitualmente equivalente. Verifica-se que, apesar de este método ser baseado em funções singulares de Green, é capaz de representar estados simples de tensões, tanto por trabalhos virtuais quanto por interpolações no domínio. Como objetivo principal deste trabalho, será demonstrado que para cada deslocamento de corpo rígido perdido, devido às restrições impostas pela simetria ou antissimetria, poderá ser utilizado um estado simples de tensão (constantes na maioria dos casos), que permitirá o estabelecimento de propriedades espectrais apropriadas. De forma a se garantir uma sistemática estruturada para o trabalho, faz-se uma abordagem de conceitos fundamentais aplicados a problemas da elastostática e potencial estacionário, na formulação variacional do Método Híbrido dos Elementos de Contorno com posteriores considerações especiais de estados simples de tensão (representados polinomialmente), para elasticidade tridimensional em geral, visto que para problemas bidimensionais o caso se torna uma particularização. Todas as combinações de simetria e antissimetria são avaliadas com a implementação numérica. Diversos exemplos de problemas bidimensionais ilustram a formulação teórica. / [en] The boundary element methods are suited for the analysis of symmetric and antisymmetric problems - in which only a part (half, quadrant or octant) of the structure needs to be explicitly considered - since, as an additional advantage when compared with a domain discretization method, no interpolation is required along the symmetry axes (for 2D problems) or planes (for 3D problems) and, consequently, no approximations are introduced thereon. Although such computational simplification may prevent some of the structures allowable rigid body movements (elasticity problems considered), this fact may be completely ignored as concerning the implementation of the traditional (collocation or Galerkin) boundary element methods. In the hybrid boundary element methods, on the other hand, special orthogonality conditions, directly or indirectly related to rigid body displacements, are required for the evaluation of elements about the main diagonal of some matrices (flexibility, displacement and stress matrices). Then, a central issue in such methods is the assessment of these matrices spectral properties for any combination of symmetry and antisymmetry and, most important, the investigation of conceptually equivalent, substitutive properties. As presented in this work, the hybrid boundary element methods, although based on singular Green s functions, are able to simulate, in terms of both virtual work and field interpolation, the simplest stress states. Then, one demonstrates that for every missing rigid body displacement - brought about by some symmetry or antisymmetry consideration - one may lay hold of a simple (in most cases constant) stress state, which enables establishing appropriate spectral properties. This work introduces the underlying variational concepts of the hybrid boundary element method and outlines the special consideration of simple (polynomial) stress states, as generally formulated for 3D elasticity, since 2D elasticity and problems of potential may be dealt with as particular cases. All combinations of symmetry and antisymmetry are outlined with the aim of numerical implementation. A series of 2D examples for problems of potential illustrate the theoretical

The study of integral operators on spaces of analytic functions has been considered for the past few decades. However, most of the studies in this line are based on spaces of analytic functions of the unit disc. For the analytic spaces of the upper half-plane, the literature is still scanty. Most notable is the recent work of Siskakis and Arvanitidis concerning the classical Ces`aro operator on Hardy spaces of the upper half-plane. In this dissertation, we characterize all continuous one-parameter groups of automorphisms of the upper halfplane according to the nature and location of their fixed points into three distinct classes, namely, the scaling, the translation, and the rotation groups. We then introduce the associated groups of weighted composition operators on both Hardy and weighted Bergman spaces of the half-plane. Interestingly, it turns out that these groups of composition operators form three strongly continuous groups of isometries. A detailed analysis of each of these groups of isometries is carried out. Specifically, we determine the spectral properties of the generators of every group, and using both spectral and semigroup theory of Banach spaces, we obtain concrete representations of the resolvents as integral operators on both Hardy and Bergman spaces of the half-plane. For the scaling group, the resulting resolvent operators are exactly the Ces`aro-like operators. The spectral properties of the obtained integral operators is also determined. Finally, we detail the theory of both Szeg¨o and Bergman projections of the half-plane, and use it to determine the duality properties of these spaces. Consequently, we obtain the adjoints of the resolvent operators on the reflexive Hardy and Bergman spaces of the half-plane.

[pt] Este trabalho apresenta técnicas de reordenação para minimização de banda, perfil e frente de malhas de elementos finitos. Um conceito unificado relacionando as malhas de elementos finitos, os grafos associados e as matrizes correspondentes é proposto. As informações geométricas, disponíveis nos programans de elemnetos finitos, são utilizadas para aumentar a eficiência dos algoritmos heurísticos. Com base nestas idéias, os algoritmos são classificados em topológicos, geométricos, híbridos e espectrais. Um Grafo de Elementos Finitos - Finite Element Graph (FEG)- é definido coo um grafo nodal(G), um garfo dual(G) ou um grafo de comunicação(G.), associado a uma dada malha de elementos finitos. Os algoritmos topológicos mais utilizados na literatura técnica, tais como, Reverse- CuthiiMcKee (RCM), Collins, Gibbs-Poole-Stockmeyer(GPS), Gibbs-King (GK), Snay e Sloan, são inventigados detalhadamente. Em particular, o algoritmo de Collins é estendido para consideração de componentes não conexos nos grafos associados e a numeração é invertida para uma posterior redução do perfil das matrizer correspondentes. Essa nova versão é denominada Modified Reverse Collins (MRCollins). Um algoritmo puramente geométrico, denominado Coordinate Based Bandwidth and Profile Reduction (CBBPR), é apresentado. Um novo algoritmo híbrido (HybWP) para redução de frente e perfil é proposto. A matriz Laplaciana [L(G), L(G) ou L (G.)], utilizada no estudo de propriedades espectrais de grafos, é construída a partir das relações usuais de adjacências entre vértices e arestas. Um algoritmo automático, baseado em propriedades espectrais de FEGs, é proposto para reordenação de nós e/ou elementos das malhas associadas. Este algoritmo, denominado Spectral FEG Resequencing (SFR), utiliza informações globais do grafo; não depende da escolha de um vértice pseudo- periférico; e não utiliza o conceito de estrutura de níveis. Um novo algoritmo espectral para determinação de vértices pseudo-periféricos em grafos também é proposto. Os algoritmos apresentados neste trabalho são implementados computacionalmente e testados utilizando- se diversos exemplos numéricos. Finalmente, conclusões são apresentadas e algumas sugestões para trabalhos futuros são propostas. / [en] This work presents resequencing techniques for minimizing bandwidth, profile and wavefront of finite element meshes. A unified approach relating a finite element mesh, its associated graphs, and the corresponding matrices is proposed. The geometrical information available from conventional finite element program is also used in order to improve heuristic algorithms. Following these ideas, the algorithms are classified here as a nodal graph (G), a dual graph (G) or a communication graph (G.) associated with a generic finie element mesh. The most widely used topological algorithms, such as Reverse-Cuthill-McKee (RCM), Collins, Gibbs-Poole-Stockmeyer (GPS), Gibbs-King (GK), Snay, and Sloan, are investigated in detail. In particular, the Collins algorithm is extended to consider nonconnected components in associated graph and the ordering provide by this algorithm is reverted for improved profile. This new version is called Modified Reverse Collins (MRCollins). A purely geometrical algorithm, called Coordinate Based Bandwidth and Profile Reduction (CBBPR), is presented. A new hybrid reordering algorithm (HybWP) for wavefront and profile reduction is proposed. The Laplacian matrix [L(G), L(G) or L(G.)], used for the study of spectral properties of an FEG, is constructed from usual vertex and edge conectivities of a graph. An automatic algorithm, based on spectral properties of an FEG, is proposed to reorder the nodes and/or elements of the associated finite element meshes. The new algorithm, called Spectral FEG Resequencing (SFR), uses global information in the graph; it does not depende on a pseudoperipheral vertex in the resequencing process; and it does not use any kind of level structure of the graph. A new spectral algorithm for finding pseudoperipheral vertices in graphs is also proposed. The algorithmpresented herein are computationally implemented and tested against several numerical examples. Finally, conclusions are drawn and directions for futue work are given.

La télédétection des végétaux, technique qui se base sur les signatures spectrales des plantes, s’avère être un outil formidable pour réaliser des inventaires de biodiversité végétale : il est maintenant possible d’identifier les plantes, de cartographier les traits fonctionnels des végétaux ou d'étudier l’impact des activités humaines à grande échelle. Cependant, il faut approfondir les connaissances sur les patrons de variation spectrale causés par les facteurs environnementaux et la phénologie. En effet, la variation spatiale et temporelle des spectres foliaires est source d’incertitude et d’erreurs lors de l’application des modèles utilisés en télédétection des végétaux. Dans le cadre de mon projet de recherche, j’évalue l’effet de l’environnement et de la phénologie sur les propriétés spectrales de neuf espèces d'arbres décidus de l’est de l'Amérique du Nord. J’ai mesuré les signatures spectrales foliaires de ces arbres dans des sites contrastés entre le 6 juin et le 8 octobre 2018 à l’aide d’un spectroradiomètre de terrain et d’une sphère d’intégration. J’ai également mesuré seize traits fonctionnels structuraux et chimiques des plantes échantillonnées. Les résultats de mon projet démontrent que les conditions environnementales n’ont pas toujours d’effet significatif sur la variation spectrale intraspécifique; lorsque c’est le cas, les sites d’échantillonnage expliquent entre 18 et 62 % de la variation intraspécifique. La phénologie quant à elle a toujours un effet significatif sur les spectres foliaires et explique entre 30 et 51 % de la variation spectrale intraspécifique. Toutes les régions spectrales sont affectées par la variation environnementale et phénologique et tous les traits structuraux et chimiques foliaires mesurés sont liés à la variation spectrale. Bien que les conditions environnementales et la phénologie affectent les spectres foliaires, cela n’empêche pas de différencier les espèces, même celles étroitement liées phylogénétiquement, à travers les sites d’étude et le temps (précision d’identification > 92 %). Il n’est pas toujours possible d’inférer les sites ou le moment d’échantillonnage de la plante échantillonnée à l’aide de sa signature spectrale; néanmoins, la variation environnementale et phénologique des spectres foliaires est suffisante pour empêcher la transférabilité de certains modèles de classification d’espèces à des contextes dissemblables de ceux qui ont servi à les construire. En effet, un modèle n’est applicable qu’à des données spectrales de variance moindre ou similaire aux données à partir desquelles le modèle a été construit. Ainsi, mon projet précise les limites d’application des modèles utilisés en télédétection des végétaux pour les milieux tempérés et forestiers peu diversifiés. Mes résultats indiquent que la variation interspécifique est supérieure à la variation intraspécifique et qu’il est donc possible de différencier les espèces malgré l’effet de l’environnement et de la phénologie sur les spectres foliaires d’arbres des forêts tempérées, démontrant le potentiel de la télédétection des végétaux pour la réalisation d’inventaires de biodiversité végétale à grande échelle. / Remote sensing of plants emerges as a formidable tool to realize biodiversity assessments : it is now possible to identify plants, map their functional traits, and monitor human activities' impacts on the vegetation at large scales and almost continuously. However, we must develop knowledge about the spectral variation patterns caused by the environment and phenology to take full advantage of remote sensing products. Indeed, the environmental and phenological variation of foliar spectra leads to uncertainty and error in remote sensing models and applications. In my project, I assess the effects of the environment and phenology on the spectral properties of nine broadleaf tree species from temperate forests of North America. I measured these species' leaf spectra across contrasting sites between the 6th of June to the 8th of October 2018 using a field spectroradiometer and an integrating sphere. I also measured sixteen structural and chemical leaf functional traits on the sampled plants. My results show that the environmental conditions do not always significantly affect the intraspecific spectral variation; when it does, the sampling sites explain between 18 and 58 % of this variation. As for phenology, it always has a significant impact on leaf spectra and explains between 30 and 51 % of the intraspecific spectral variation. Every spectral region is affected by environmental and phenological variation, and all structural and chemical traits measured are linked to the spectral variation. Although the environmental conditions and phenology affect leaf spectra, it does not hinder our ability to differentiate tree species, even those closely related, across sites and time (identification accuracy > 92 %). Besides, it is not always possible to infer a plant's sampling site or date from its spectra. Nevertheless, foliar spectra's environmental and phenological variation is sufficient to limit classification models' transferability. My results show that a model applies only to spectral data of lesser or comparable variance to the data used to train the model. Therefore, my project helps define the application limits of models used in plant remote sensing in temperate and low diversity forest systems. It also demonstrates that interspecific spectral variation is greater than intraspecific spectral variation, and thus we can discriminate species despite the effects of the environment and phenology on the leaf spectra of temperate forest trees, validating the potential of vegetation remote sensing to conduct biodiversity assessments at spatial and temporal scales previously unattainable.

Toute perturbation dans les systèmes linéaires peut gravement dégrader la performance des méthodes itératives lorsque les directions conjuguées sont constituées. Ce problème peut être partiellement résolu par les méthodes du gradient à retards, qui ne garantissent pas la descente de la fonction quadratique, mais peuvent améliorer la convergence par rapport aux méthodes traditionnelles. Les travaux ultérieurs se sont concentrés sur les méthodes du gradient alternées avec deux ou plusieurs types de pas afin d'interrompre le zigzag. Des papiers récents ont suggéré que la révélation d'information de second ordre avec des pas à retards pourrait réduire de manière asymptotique les espaces de recherche dans des dimensions de plus en plus petites. Ceci a conduit aux méthodes du gradient avec alignement dans lesquelles l'étape essentielle et l'étape auxiliaire sont effectuées en alternance. Des expériences numériques ont démontré leur efficacité. Cette thèse considère d'abord des méthodes du gradient efficaces pour résoudre les systèmes linéaires symétriques définis positifs. Nous commençons par étudier une méthode alternée avec la propriété de terminaison finie à deux dimensions. Ensuite, nous déduisons davantage de propriétés spectrales pour les méthodes du gradient traditionnelles. Ces propriétés nous permettent d’élargir la famille de méthodes du gradient avec alignement et d’établir la convergence de nouvelles méthodes. Nous traitons également les itérations de gradient comme un processus peu coûteux intégré aux méthodes de splitting. En particulier, nous abordons le problème de l’estimation de paramètre et suggérons d’utiliser les méthodes du gradient rapide comme solveurs internes à faible précision. Dans le cas parallèle, nous nous concentrons sur les formulations avec retards pour lesquelles il est possible de réduire les coûts de communication. Nous présentons également de nouvelles propriétés et méthodes pour les itérations de gradient s-dimensionnelles. En résumé, cette thèse s'intéresse aux trois sujets interreliés dans lesquelles les itérations de gradient peuvent être utilisées en tant que solveurs efficaces, qu’outils intégrés pour les méthodes de splitting et que solveurs parallèles pour réduire la communication. Des exemples numériques sont présentés à la fin de chaque sujet pour appuyer nos résultats théoriques. / Any perturbation in linear systems may severely degrade the performance of iterative methods when conjugate directions are constructed. This issue can be partially remedied by lagged gradient methods, which does not guarantee descent in the quadratic function but can improve the convergence compared with traditional gradient methods. Later work focused on alternate gradient methods with two or more steplengths in order to break the zigzag pattern. Recent papers suggested that revealing of second-order information along with lagged steps could reduce asymptotically the search spaces in smaller and smaller dimensions. This led to gradient methods with alignment in which essential and auxiliary steps are conducted alternately. Numerical experiments have demonstrated their effectiveness. This dissertation first considers efficient gradient methods for solving symmetric positive definite linear systems. We begin by studying an alternate method with two-dimensional finite termination property. Then we derive more spectral properties for traditional steplengths. These properties allow us to expand the family of gradient methods with alignment and establish the convergence of new methods. We also treat gradient iterations as an inexpensive process embedded in splitting methods. In particular we address the parameter estimation problem and suggest to use fast gradient methods as low-precision inner solvers. For the parallel case we focus on the lagged formulations for which it is possible to reduce communication costs. We also present some new properties and methods for s-dimensional gradient iterations. To sum up, this dissertation is concerned with three inter-related topics in which gradient iterations can be employed as efficient solvers, as embedded tools for splitting methods and as parallel solvers for reducing communication. Numerical examples are presented at the end of each topic to support our theoretical findings.

Cette thèse est dédiée à l'étude d'opérateurs de composition pondérés sur plusieurs espaces fonctionnels sous fond du problème du sous-espace invariant. Cet important problème ouvert pose la question de l'existence pour tout opérateur sur un espace de Hilbert, complexe, séparable de dimension infinie, d'un sous-espace fermé, non-trivial et invariant par cet opérateur. La première partie est consacrée à l'étude spectrale et à la caractérisation des vecteurs cycliques d'un opérateur de composition à poids particulier sur L^2([0,1]^d) : l'opérateur de type Bishop, introduit comme possible contre-exemple au problème du sous-espace invariant. Les seconde, troisième et quatrième parties abordent ce problème sous un autre aspect : celui de l'universalité d'un opérateur. Ces opérateurs universels possèdent la propriété d'universalité : la description complète des sous-espaces invariants d'un opérateur universel permettrait de répondre au problème du sous-espace invariant. Déterminer l'universalité d'un opérateur repose sur l'établissement de propriétés spectrales fines de l’opérateur considéré (théorème de Caradus). Dans ce but, nous établissons des propriétés spectrales ad-hoc de classes d’opérateurs de composition à poids sur L^2([0,1]), les espaces de Sobolev d’ordre n, sur les espaces de Hardy du disque unité et du demi-plan supérieur, permettant de déduire des résultats d’universalité. Nous obtenons aussi une généralisation du critère d’universalité. Dans la dernière partie, nous donnons une caractérisation des opérateurs de composition rsid16415432 inversibles et une caractérisation partielle des opérateurs de composition isométriques sur les espaces de Hardy de l’anneau / In this thesis, we study classes of weighted composition operators on several functional spaces in the background of the invariant subspace problem. This important open problem asks the question of the existence for every operator, defined on a complex and separable infinite dimensional Hilbert space, of a non trivial closed subspace invariant under the operator. The first part is dedicated to the establishment of the spectrum and the characterization of cyclic vectors of a special weighted composition operator defined on L^2([0,1]^d) : the Bishop type operator, introduced as possible counter-example of the invariant subspace problem. The second, third and fourth part approach the problem from the point of view of universal operators. More precisely, universal operators have the universal property in the sense of the complete description of all the invariant subspaces of a universal operator could solve the invariant subspace problem. A sufficient condition for an operator to be universal (Caradus’theorem) is given in terms of spectral properties. To this aim, we establish ad-hoc spectral properties of a class of weighted composition operators on L^2([0,1]) and Sobolev spaces of order n, of composition operator in the Hardy space of the unit disc and of the upper half-plane, which lead us to deduce universality results. We also obtain a generalization of the universality criteria mentioned above. In the last part, we give a characterization of invertible composition operators and a partial characterization of composition operators on the Hardy space of the annulus