This Master's thesis will develops a modern approach to complex interpolation problems studied by Carath\'odory, Nevanlinna, Pick, and Schur in the early $20^{th}$ century. The fundamental problem to solve is as follows: given complex numbers $z_1,z_2,...,z_N$ of modulus at most $1$ and $w_1,w_2,...,w_N$ additional complex numbers, what is a necessary and sufficiency condition for the existence of an analytic function $f: \mathbb{D} \rightarrow \mathbb$ satisfying $f(z_i) = w_i$ for $1 \leq i \leq N$ and $\vert f(z) \vert \leq 1$ for each $z \in \mathbb{D}$? The key idea is to realize bounded, analytic functions (the algebra $H^\infty$) as the \emph of the Hardy class of analytic functions, and apply dilation theory to this algebra. This operator theoretic approach may then be applied to a wider class of interpolation problems, as well as their matrix-valued equivalents. This also yields a fundamental distance formula for $H^\infty$, which provides motivation for the study of completely isometric representations of certain quotient algebras. Our attention is then turned to a related interpolation problem. Here we require the interpolating function $f$ to satisfy the additional property $f'(0) = 0$. When $z_i =0$ for some $i$, we arrive at a special case of a problem class studied previously. However, when $0$ is not in the interpolating set, a significant degree of complexity is inherited. The dilation theoretic approach employed previously is not effective in this case. A more function theoretic viewpoint is required, with the proof of the main interpolation theorem following from a factorization lemma for the Hardy class of analytic functions. We then apply the theory of completely isometric maps to show that matrix interpolation fails when one imposes this constraint.

This Master's thesis will develops a modern approach to complex interpolation problems studied by Carath\'odory, Nevanlinna, Pick, and Schur in the early $20^{th}$ century. The fundamental problem to solve is as follows: given complex numbers $z_1,z_2,...,z_N$ of modulus at most $1$ and $w_1,w_2,...,w_N$ additional complex numbers, what is a necessary and sufficiency condition for the existence of an analytic function $f: \mathbb{D} \rightarrow \mathbb$ satisfying $f(z_i) = w_i$ for $1 \leq i \leq N$ and $\vert f(z) \vert \leq 1$ for each $z \in \mathbb{D}$? The key idea is to realize bounded, analytic functions (the algebra $H^\infty$) as the \emph of the Hardy class of analytic functions, and apply dilation theory to this algebra. This operator theoretic approach may then be applied to a wider class of interpolation problems, as well as their matrix-valued equivalents. This also yields a fundamental distance formula for $H^\infty$, which provides motivation for the study of completely isometric representations of certain quotient algebras. Our attention is then turned to a related interpolation problem. Here we require the interpolating function $f$ to satisfy the additional property $f'(0) = 0$. When $z_i =0$ for some $i$, we arrive at a special case of a problem class studied previously. However, when $0$ is not in the interpolating set, a significant degree of complexity is inherited. The dilation theoretic approach employed previously is not effective in this case. A more function theoretic viewpoint is required, with the proof of the main interpolation theorem following from a factorization lemma for the Hardy class of analytic functions. We then apply the theory of completely isometric maps to show that matrix interpolation fails when one imposes this constraint.

Thèse (M.Sc.)--Université Laval, 2007. / Bibliogr.: f. [72]. Publié aussi en version électronique dans la Collection Mémoires et thèses électroniques.

Cette thèse donne de nouveaux résultats de lois infiniment divisibles. La résolution d'une conjecture de Steutel (1973) à propos de la divisibilité infinie des puissances d'une variable gamma, et d'une conjecture de Bondesson (1992) à propos de la monotonicité complète hyperbolique des densités stables positives en sont les deux résultats principaux. Des fonctions spéciales (fonctions de Bessel, hypergéométriques, de Mittag-Leffler) apparaissent régulièrement tout au long du manuscrit. / In this thesis, we give some new results of infinite divisibility on the half-line. The main results are : - The resolution of a conjecture due to Steutel (1973) about the infinite divisibility of negative powers of a gamma variable.- The resolution of a conjecture due to Bondesson (1992) concerning stable densities and hyperbolic complete monotonicity property.

An operator-theoretic formulation of the interpolation problem posed by Nevanlinna and Pick in the early twentieth century asks for conditions under which there exists a multiplier of a reproducing kernel Hilbert space that interpolates a specified set of data. Paul S. Muhly and Baruch Solel have shown that their theory for operator algebras built from W*-correspondences provides an appropriate context for generalizing this classic question. Their reproducing kernel W*-correspondences are spaces of functions that generalize the reproducing kernel Hilbert spaces. Their Nevanlinna-Pick interpolation theorem, which is proved using commutant lifting, implies that the algebra of multipliers of the reproducing kernel W*-correspondence associated with a certain W*-version of the classic Szegö kernel may be identified with their primary operator algebra of interest, the Hardy algebra. To provide a context for generalizing another familiar topic in operator theory, the study of the weighted Hardy spaces, Muhly and Solel have recently expanded their theory to include operator-valued weights. This creates a new family of reproducing kernel W*-correspondences that includes certain, though not all, classic weighted Hardy spaces. It is the purpose of this thesis to generalize several of Muhly and Solel's results to the weighted setting and investigate the function-theoretic properties of the resulting spaces. We give two principal results. The first is a weighted version of Muhly and Solel's commutant lifting theorem, which we obtain by making use of Parrott's lemma. The second main result, which in fact follows from the first, is a weighted Nevanlinna-Pick interpolation theorem. Other results, several of which follow from the two primary results, include the construction of an orthonormal basis for the nonzero tensor product of two W*-corrrespondences, a double commutant theorem, the identification of several function-theoretic properties of the elements in the reproducing kernel W*-correspondence associated with a weighted W*-Szegö kernel as well as the elements in its algebra of mutlipliers, and the presentation of a relationship between this algebra of multipliers and a weighted Hardy algebra. In addition, we consider a candidate for a W*-version of the complete Pick property and investigate the aforementioned weighted W*-Szegö kernel in its light.

The classical Nevanlinna-Pick interpolation theorem, proved in 1915 by Pick and in 1919 by Nevanlinna, gives a condition for when there exists an interpolating function in H∞(D) for a specified set of data in the complex plane. In 1967, Sarason proved his commutant lifting theorem for H∞(D), from which an operator theoretic proof of the classical Nevanlinna-Pick theorem followed. Several competing noncommutative generalizations arose as a consequence of Sarason's result, and two strategies emerged for proving generalized Nevanlinna-Pick theorems: via a commutant lifting theorem or via a resolvent, or displacement, equation. We explore the difference between these two approaches. Specifically, we compare two theorems: one by Constantinescu-Johnson from 2003 and one by Muhly-Solel from 2004. Muhly-Solel's theorem is stated in the highly general context of W*-correspondences and is proved via commutant lifting. Constantinescu-Johnson's theorem, while stated in a less general context, has the advantage of an elegant proof via a displacement equation. In order to make the comparison, we first generalize Constantinescu-Johnson's theorem to the setting of W*-correspondences in Theorem 3.0.1. Our proof, modeled after Constantinescu-Johnson's, hinges on a modified version of their displacement equation. Then we show that Theorem 3.0.1 is fundamentally different from Muhly-Solel's. More specifically, interpolation in the sense of Muhly-Solel's theorem implies interpolation in the sense of Theorem 3.0.1, but the converse is not true. Nevertheless, we identify a commutativity assumption under which the two theorems yield the same result. In addition to the two main theorems, we include smaller results that clarify the connections between the notation, space of interpolating maps, and point evaluation employed by Constantinescu-Johnson and those employed by Muhly-Solel. We conclude with an investigation of the relationship between Theorem 3.0.1 and Popescu's generalized Nevanlinna-Pick theorem proved in 2003.

In this dissertation, we solve multivariable Nevanlinna-Pick type interpolation problems. Particularly, we consider the left tangential interpolation problems on the commutative or noncommutative unit ball. For the commutative setting, we discuss left-tangential operator-argument interpolation problems for Schur-class multipliers on the Drury-Arveson space and for the noncommutative setting, we discuss interpolation problems for Schur-class multipliers on Fock space. We apply the Krein-space geometry approach (also known as the Grassmannian Approach). To implement this approach J-versions of Beurling-Lax representers for shift-invariant subspaces are required. Here we obtain these J-Beurling-Lax theorems by the state-space method for both settings. We see that the Krein-space geometry method is particularly simple in solving the interpolation problems when the Beurling-Lax representer is bounded. The Potapov approach applies equally well whether the representer is bounded or not. / Ph. D.

Tableau d’honneur de la Faculté des études supérieures et postdoctorales, 2015-2016 / Les systèmes de Schwarz-Pick sont de puissants outils qui permettent d'enrichir l'étude de la géométrie des domaines de l'espace à plusieurs variables complexes. Plus particulièrement, les pseudodistances de Carathéodory et de Kobayashi forment respectivement le plus grand et le plus petit système. L'objet de cet ouvrage consiste à regrouper et synthétiser les recherches autour du calcul de ces pseudodistances sur le bidisque symétrisé. Il s'agit d'un domaine de l'espace à deux variables complexes qui possède une géométrie riche et qui joue un rôle clé dans la résolution du problème de Nevanlinna-Pick spectral. Sur le bidisque symétrisé, il est possible de calculer explicitement la pseudodistance de Carathéodory par le biais de la théorie des opérateurs. Le calcul de la pseudodistance de Kobayashi, se fera elle à travers un problème d'interpolation du disque unité avec des valeurs cibles dans le bidisque symétrisé, résolu à l'aide du théorème de Nevanlinna-Pick classique.

Tableau d’honneur de la Faculté des études supérieures et postdoctorales, 2007-2008. / Le but de cet ouvrage est de montrer, grâce à l'introduction d'éléments de théorie; géométrique, comment il est possible d'apporter de nouvelles idées à, la résolution d'un problème: d'interpolation connu sous le nom de problème, classique de Nevanlinna Pick et qui s'énonce comme suit : étant donné n points distincts z^,. . . , zn et n points W\,...,wn tous appartenant au disque unité D, déterminer des conditions suffisantes et nécessaires assurant l'existence d'une fonction analytique / : D —> D satisfaisant /(z,) = m, pour /' = 1, . . . , n. Une solution complète fut apportée d'abord par Pick en 1916 et indépendamment par Nevanlinna en 1919. Une toute nouvelle approche sera donc présentée dans ce travail utilisant la géométrie hyperbolique, de même qu'une version à points multiples du lemine de Schwarz-Pick.

Le problème de l'adaptation d'impédance en ingénierie des hyper fréquences et en électronique en général consiste à minimiser la réflexion de la puissance qui doit être transmise, par un générateur, à une charge donnée dans une bande de fréquence. Les exigences d'adaptation et de filtrage dans les systèmes de communication classiques sont généralement satisfaites en utilisant un circuit d'adaptation suivi d'un filtre. Nous proposons ici de concevoir des filtres d'adaptation qui intègrent à la fois les exigences d'adaptation et de filtrage dans un seul appareil et augmentent ainsi l'efficacité globale et la compacité du système. Dans ce travail, le problème d'adaptation est formulé en introduisant un problème d'optimisation convexe dans le cadre établi par la théorie de d'adaptation de Fano et Youla. De ce contexte, au moyen de techniques modernes de programmation semi-définies non linéaires, un problème convexe, et donc avec une optimalité garantie, est obtenu. Enfin, pour démontrer les avantages fournis par la théorie développée au-delà de la synthèse de filtres avec des charges complexes variables en fréquence, nous examinons deux applications pratiques récurrentes dans la conception de ce type de dispositifs. Ces applications correspondent, d'une part, à l'adaptation d'un réseau d'antennes dans le but de maximiser l'efficacité du rayonnement, et, d'autre part, à la synthèse de multiplexeurs où chacun des filtres de canal est adapté au reste du dispositif, notamment les filtres correspondant aux autres canaux. / The problem of impedance matching in electronics and particularly in RF engineering consists on minimising the reflection of the power that is to be transmitted, by a generator, to a given load within a frequency band. The matching and filtering requirements in classical communication systems are usually satisfied by using a matching circuit followed by a filter. We propose here to design matching filters that integrate both, matching and filtering requirements, in a single device and thereby increase the overall efficiency and compactness of the system. In this work, the matching problem is formulated by introducing convex optimisation on the framework established by the matching theory of Fano and Youla. As a result, by means of modern non-linear semi-definite programming techniques, a convex problem, and therefore with guaranteed optimality, is achieved. Finally, to demonstrate the advantages provided by the developed theory beyond the synthesis of filters with frequency varying loads, we consider two practical applications which are recurrent in the design of communication devices. These applications are, on the one hand, the matching of an array of antennas with the objective of maximizing the radiation efficiency, and on the other hand the synthesis of multiplexers where each of the channel filters is matched to the rest of the device, including the filters corresponding to the other channels.