The generalized Harish-Chandra homomorphism for Hecke algebras of real reductive Lie groups

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mathematics, 2005.Includes bibliographical references (p. 73-74).For complex reductive Lie algebras g, the classical Harish-Chandra homomorphism allows to link irreducible finite dimensional representations of g to those of certain subalgebras l. The Casselman-Osborne theorem establishes an extension of this link to infinite dimensional irreducible representations. In this paper we present a generalized Harish-Chandra homomorphism construction for Hecke algebras, and establish the corresponding generalized Casselman-Osborne theorem. This homomorphism can be used to link representations of (g, L n K)-pairs to those of (g, L n K)-pairs, where is a certain subalgebra of g as in the classical case. Since representations of such pairs are closely related to those of the underlying Lie group G, this construction is a good first approximation to lifting the Harish-Chandra homomorphism from the Lie algebra to the Lie group level.by Karen Bernhardt.S.M

Mathematical analysis of models of non-homogeneous fluids and of hyperbolic equations with low regularity coefficients

The present thesis is devoted to the study both of strictly hyperbolic operators with low regularity coefficients and of the density-dependent incompressible Euler system. On the one hand, we show a priori estimates for a second order strictly hyperbolic operator whose highest order coefficients satisfy a log-Zygmund continuity condition in time and a log-Lipschitz continuity condition with respect to space. Such an estimate involves a time increasing loss of derivatives..

Hodge cohomology of negatively curved manifolds

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mathematics, 1986.MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE.Bibliography: leaves 181-183.by Rafe Roys Mazzeo.Ph.D

Nahm’s equation and the search for classical solutions in Yang-Mills theory

The history of the theory of magnetic monopoles in classical electrodynamics and unified gauge theories is reviewed, and the Atiyah-Ward and Atiyah-Drinfe1d-Hitchin-Man in constructions of exact classical solutions to the self-dual Yang-Mills equations are described. It is shown that the one-dimensional self-dual equation introduced by Nahm can be reformulated as a Rieraann-Hi1bert problem through the twister transform previously used by Ward for monopole and instanton fields, and a general formula for the patching matrix is derived. This is evaluated in some special cases, and a few simple examples are given where Nahm's equation can be solved by this method. An attempt is made to generalize the ADHM construction to treat non self dual Yang-Mills fields, with only partial success. The one-dimensional analogue of the second-order Yang-Mills equation, the so-called non self dual Nahm equation, is investigated, paying particular attention to a simple ansatz in which translation of the fields is equivalent to a mere scale transformation of the matrices T(_i)(Z). For these 'separable solutions' the matrices satisfy certain cubic equations, whose solution space depends critically on the nature of the Lie algebra under consideration. It is shown that corresponding to certain Riemannian symmetric pairs there are one-parameter families of 'interpolating solutions' to the cubic equations, which join oppositely oriented bases of a Lie subalgebra. The associated matrix-valued functions T(_i)(z) therefore interpolate between solutions of 'selfdual' and 'antiselfdual' Nahm equations

Analyse mathématique des modèles de fluids non-homogènes et d'équations hyperboliques à coefficients peu réguliers

Cette thèse est consacrée à l'étude des opérateurs strictement hyperboliques à coefficients peu réguliers, aussi bien qu'à l'étude du système d'Euler incompressible à densité variable. Dans la première partie, on montre des estimations a priori pour des opérateurs strictement hyperboliques dont les coefficients d'ordre le plus grand satisfont une condition de continuité log-Zygmund par rapport au temps et une condition de continuité log-Lipschitz par rapport à la variable d'espace. Ces estimations comportent une perte de dérivées qui croît en temps. Toutefois, elles sont suffisantes pour avoir encore le caractère bien posé du problème de Cauchy associé dans l'espace H^inf (pour des coefficients du deuxième ordre ayant assez de régularité).Dans un premier temps, on considère un opérateur complet en dimension d'espace égale à 1, dont les coefficients du premier ordre étaient supposés hölderiens et celui d'ordre 0 seulement borné. Après, on traite le cas général en dimension d'espace quelconque, en se restreignant à un opérateur de deuxième ordre homogène: le passage à la dimension plus grande exige une approche vraiment différente. Dans la deuxième partie de la thèse, on considère le système d'Euler incompressible à densité variable. On montre son caractère bien posé dans des espaces de Besov limites, qui s'injectent dans la classe des fonctions globalement lipschitziennes, et on établit aussi des bornes inférieures pour le temps de vie de la solution ne dépendant que des données initiales. Cela fait, on prouve la persistance des structures géométriques, comme la régularité stratifiée et conormale, pour les solutions de ce système. À la différence du cas classique de densité constante, même en dimension 2 le tourbillon n'est pas transporté par le champ de vitesses. Donc, a priori on peut s'attendre à obtenir seulement des résultats locaux en temps. Pour la même raison, il faut aussi laisser tomber la structure des poches de tourbillon. La théorie de Littlewood-Paley et le calcul paradifférentiel nous permettent d'aborder ces deux différents problèmes. En plus, on a besoin aussi d'une nouvelle version du calcul paradifférentiel, qui dépend d'un paramètre plus grand que ou égal à 1, pour traiter les opérateurs à coefficients peu réguliers. Le cadre fonctionnel adopté est celui des espaces de Besov, qui comprend en particulier les ensembles de Sobolev et de Hölder. Des classes intermédiaires de fonctions, de type logarithmique, entrent, elles aussi, en jeuThe present thesis is devoted both to the study of strictly hyperbolic operators with low regularity coefficients and of the density-dependent incompressible Euler system. On the one hand, we show a priori estimates for a second order strictly hyperbolic operator whose highest order coefficients satisfy a log-Zygmund continuity condition in time and a log-Lipschitz continuity condition with respect to space. Such an estimate involves a time increasing loss of derivatives. Nevertheless, this is enough to recover well-posedness for the associated Cauchy problem in the space $H^infty$ (for suitably smooth second order coefficients).In a first time, we consider acomplete operator in space dimension $1$, whose first order coefficients were assumed Hölder continuous and that of order $0$only bounded. Then, we deal with the general case of any space dimension, focusing on a homogeneous second order operator: the step to higher dimension requires a really different approach. On the other hand, we consider the density-dependent incompressible Euler system. We show its well-posedness in endpoint Besov spaces embedded in the class of globally Lipschitz functions, producing also lower bounds for the lifespan of the solution in terms of initial data only. This having been done, we prove persistence of geometric structures, such as striated and conormal regularity, for solutions to this system. In contrast with the classical case of constant density, even in dimension $2$ the vorticity is not transported by the velocity field. Hence, a priori one can expect to get only local in time results. For the same reason, we also have to dismiss the vortex patch structure. Littlewood-Paley theory and paradifferential calculus allow us to handle these two different problems .A new version of paradifferential calculus, depending on a parameter $ggeq1$, is also needed in dealing with hyperbolic operators with nonregular coefficients. The general framework is that of Besov spaces, which includes in particular Sobolev and Hölder sets. Intermediate classes of functions, of logaritmic type, come into play as wellPARIS-EST-Université (770839901) / SudocSudocFranceF

On the effect of confounding in linear regression models: an approach based on the theory of quadratic forms

The main topic of this thesis is confounding in linear regression models. It arises when a relationship between an observed process, the covariate, and an outcome process, the response, is influenced by an unmeasured process, the confounder, associated with both. Consequently, the estimators for the regression coefficients of the measured covariates might be severely biased, less efficient and characterized by misleading interpretations. Confounding is an issue when the primary target of the work is the estimation of the regression parameters. The central point of the dissertation is the evaluation of the sampling properties of parameter estimators. This work aims to extend the spatial confounding framework to general structured settings and to understand the behaviour of confounding as a function of the data generating process structure parameters in several scenarios focusing on the joint covariate-confounder structure. In line with the spatial statistics literature, our purpose is to quantify the sampling properties of the regression coefficient estimators and, in turn, to identify the most prominent quantities depending on the generative mechanism impacting confounding. Once the sampling properties of the estimator conditionally on the covariate process are derived as ratios of dependent quadratic forms in Gaussian random variables, we provide an analytic expression of the marginal sampling properties of the estimator using Carlson’s R function. Additionally, we propose a representative quantity for the magnitude of confounding as a proxy of the bias, its first-order Laplace approximation. To conclude, we work under several frameworks considering spatial and temporal data with specific assumptions regarding the covariance and cross-covariance functions used to generate the processes involved. This study allows us to claim that the variability of the confounder-covariate interaction and of the covariate plays the most relevant role in determining the principal marker of the magnitude of confounding

Some conditions for a given positive integer to be contained in a sum of sets of nonnegative integers

In this thesis, we study conditions not involving density which guarantee that a given positive integer is contained in a sum of sets of nonnegative integers. We survey the literature, give more detailed proofs of some known theorems, develop some new theorems, and make some conjectures