The purpose of this paper is to investigate the nature and practical uses of Chebyshev polynomials. Chapter I gives recognition to mathematicians responsible for studies in this area. Chapter II enumerates several mathematical situations in which the polynomials naturally arise and suggests reasons for the pursuance of their study. Chapter III includes: Chebyshev polynomials as related to "best" polynomial approximation, Chebyshev series, and methods of producing polynomial approximations to continuous functions. Chapter IV discusses the use of Chebyshev polynomials to solve certain differential equations and Chebyshev-Gauss quadrature.

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2014-10-31T11:26:53Z No. of bitstreams: 2 Dissertação - Emílio Curi Neto - 2014.pdf: 3380066 bytes, checksum: d90415ab2be40c912a8f3437adf514bb (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2014-10-31T13:45:48Z (GMT) No. of bitstreams: 2 Dissertação - Emílio Curi Neto - 2014.pdf: 3380066 bytes, checksum: d90415ab2be40c912a8f3437adf514bb (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2014-10-31T13:45:48Z (GMT). No. of bitstreams: 2 Dissertação - Emílio Curi Neto - 2014.pdf: 3380066 bytes, checksum: d90415ab2be40c912a8f3437adf514bb (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Previous issue date: 2014-07-03 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this work the main goal is focused on applying the theory of Taylor polynomial approximations applied on the trigonometric function defined by f : [0; 2 ] 􀀀! R, where f(x) = sin(x). To achieve this goal, eight sections were developed, in which initially a reflection on the problem and the need to obtain the values in this respect in that it is wide angle measure x is presented. Is presented and subsequently treated a problem involving the movement of a pendulum, which uses the approximation sin(x) x where x belongs to a certain range. In the sections that follow a literature review of the theories of differential and integral calculus is presented, and the related theory of Taylor approximation of functions by polynomials. Later we used these theories to analyze and determine polynomials approximating the function f(x) = sin(x) in a neighborhood of the point x = 0, and estimate the error when we applied these approaches. At this time the error occurred due to the approach used in the pendulum problem was also analyzed. Finally a hint of practice to be held in the classroom using the theories treated here as well as the study of the problem of heat transfer in a bar through the theory of Fourier activity is presented. / Neste trabalho o objetivo principal está focado em aplicar a teoria de Taylor relativa à aproximações polinomiais aplicadas à função trigonométrica definida por f : [0; 2 ] 􀀀! R, onde f(x) = sen(x). Para alcançar esse objetivo, foram desenvolvidas oito seções, nas quais inicialmente é apresentada uma reflexão sobre a necessidade e a problemática de obtêr-se os valores desta relação a medida em que varia-se a medida do ângulo x. Posteriormente é apresentado e tratado um problema envolvendo o movimento de um pêndulo, o qual utiliza a aproximação sen(x) x onde x pertence o um certo intervalo. Nas seções que seguem é apresentada uma revisão bibliográfica das Teorias do Cálculo Diferencial e Integral, assim como da Teoria de Taylor relacionada à aproximação de funções através de polinômios. Posteriormente utilizou-se estas teorias para analisar e determinar polinômios que aproximam a função sen(x) em uma vizinhança do ponto x = 0, assim como estimar o erro gerado ao utilizar-se estas aproximações. Nesse momento também foi analisado o erro ocorrido devido à aproximação utilizada no problema do pêndulo. Por fim é apresentada uma sugestão de atividade prática a ser realizada em sala de aula utilizando as teorias aqui tratadas, assim como o estudo do problema de transferência de calor em uma barra através da teoria de Fourier.

Cette thèse est dédiée à l'étude de la hiérarchie moments-sommes-de-carrés, une famille de problèmes de programmation semi-définie en optimisation polynomiale, couramment appelée hiérarchie de Lasserre. Nous examinons différents aspects de ses propriétés et applications. Comme application de la hiérarchie, nous approchons certains objets potentiellement compliqués, comme l'abscisse polynomiale et les plans d'expérience optimaux sur des domaines semi-algébriques. L'application de la hiérarchie de Lasserre produit des approximations par des polynômes de degré fixé et donc de complexité bornée. En ce qui concerne la complexité de la hiérarchie elle-même, nous en construisons une modification pour laquelle un taux de convergence amélioré peut être prouvé. Un concept essentiel de la hiérarchie est l'utilisation des modules quadratiques et de leurs duaux pour appréhender de manière flexible le cône des polynômes positifs et le cône des moments. Nous poursuivons cette idée pour construire des approximations étroites d'ensembles semi-algébriques à l'aide de séparateurs polynomiaux. / This thesis is dedicated to investigations of the moment-sums-of-squares hierarchy, a family of semidefinite programming problems in polynomial optimization, commonly called the Lasserre hierarchy. We examine different aspects of its properties and purposes. As applications of the hierarchy, we approximate some potentially complicated objects, namely the polynomial abscissa and optimal designs on semialgebraic domains. Applying the Lasserre hierarchy results in approximations by polynomials of fixed degree and hence bounded complexity. With regard to the complexity of the hierarchy itself, we construct a modification of it for which an improved convergence rate can be proved. An essential concept of the hierarchy is to use quadratic modules and their duals as a tractable characterization of the cone of positive polynomials and the moment cone, respectively. We exploit further this idea to construct tight approximations of semialgebraic sets with polynomial separators.

Le calcul rigoureux vise à produire des représentations certifiées pour les solutions de nombreux problèmes, notamment en analyse fonctionnelle, comme des équations différentielles ou des problèmes de contrôle optimal. En effet, certains domaines particuliers comme l’ingénierie des systèmes critiques ou les preuves mathématiques assistées par ordinateur ont des exigences de fiabilité supérieures à ce qui peut résulter de l’utilisation d’algorithmes relevant de l’analyse numérique classique.Notre objectif consiste à développer des algorithmes à la fois efficaces et validés / certifiés, dans le sens où toutes les erreurs numériques (d’arrondi ou de méthode) sont prises en compte. En particulier, nous recourons aux approximations polynomiales rigoureuses combinées avec des méthodes de validation a posteriori à base de points fixes. Ces techniques sont implémentées au sein d’une bibliothèque écrite en C, ainsi que dans un développement de preuve formelle en Coq, offrant ainsi le plus haut niveau de confiance, c’est-à-dire une implémentation certifiée.Après avoir présenté les opérations élémentaires sur les approximations polynomiales rigoureuses, nous détaillons un nouvel algorithme de validation pour des approximations sous forme de séries de Tchebychev tronquées de fonctions D-finies, qui sont les solutions d’équations différentielles ordinaires linéaires à coefficients polynomiaux. Nous fournissons une analyse fine de sa complexité, ainsi qu’une extension aux équations différentielles ordinaires linéaires générales et aux systèmes couplés de telles équations. Ces méthodes dites symboliques-numériques sont ensuite utilisées dans plusieurs problèmes reliés : une nouvelle borne sur le nombre de Hilbert pour les systèmes quartiques, la validation de trajectoires de satellites lors du problème du rendez-vous linéarisé, le calcul de polynômes d’approximation optimisés pour l’erreur d’évaluation, et enfin la reconstruction du support et de la densité pour certaines mesures, grâce à des techniques algébriques. / Rigorous numerics aims at providing certified representations for solutions of various problems, notably in functional analysis, e.g., differential equations or optimal control. Indeed, specific domains like safety-critical engineering or computer-assisted proofs in mathematics have stronger reliability requirements than what can be achieved by resorting to standard numerical analysis algorithms. Our goal consists in developing efficient algorithms, which are also validated / certified in the sense that all numerical errors (method or rounding) are taken into account. Specifically, a central contribution is to combine polynomial approximations with a posteriori fixed-point validation techniques. A C code library for rigorous polynomial approximations (RPAs) is provided, together with a Coq formal proof development, offering the highest confidence at the implementation level.After providing basic operations on RPAs, we focus on a new validation algorithm for Chebyshev basis solutions of D-finite functions, i.e., solutions of linear ordinary differential equations (LODEs) with polynomial coefficients. We give an in-depth complexity analysis, as well as an extension to general LODEs, and even coupled systems of them. These symbolic-numeric methods are finally used in several related problems: a new lower bound on the Hilbert number for quartic systems; a validation of trajectories arising in the linearized spacecraft rendezvous problem; the design of evaluation error efficient polynomial approximations; and the support and density reconstruction of particular measures using algebraic techniques.