Kernels of adjoints of composition operators on Hilbert spaces of analytic functions

This thesis contains a collection of results in the study of the adjoint of a composition operator and its kernel in weighted Hardy spaces, in particular, the classical Hardy, Bergman, and Dirichlet spaces. In 2006, Cowen and Gallardo-Gutiérrez laid the groundwork for an explicit formula for the adjoint of a composition operator with rational symbol acting on the Hardy space, and in 2008, Hammond, Moorhouse, and Robbins established such a formula. In 2014, Goshabulaghi and Vaezi obtained analogous formulas for the adjoint of a composition operator in the Bergman and Dirichlet spaces. While it is known that the kernel of the adjoint of a composition operator whose symbol is not univalent on the complex unit disk is infinite-dimensional, no classification has been given for functions in this kernel. Chapter 1 introduces the relevant definitions in the study of composition operators and their adjoints. Chapter 2 provides the background for results obtained by Cowen and Gallardo-Gutiérrez, and Hammond, Moorhouse, and Robbins in the Hardy space. The results by Goshabulaghi and Vaezi for the Bergman and Dirichlet spaces are also given. Chapter 3 contains explicit descriptions of the kernel of the adjoint of a composition operator in a particular class on general weighted Hardy spaces. Chapter 4 uses the adjoint formula by Hammond, Moorhouse, and Robbins to give a functional equation that characterizes functions in the kernel of the adjoint of a composition operator with a rational symbol of degree two on the Hardy space. Chapters 5 and 6 use the adjoint formulas by Goshabulaghi and Vaezi to prove some results about the kernels of adjoints of composition operators on the Bergman and Dirichlet spaces

Holomorphic Semiflows and Poincaré-Steklov Semigroups

Die Arbeit untersucht einen überraschenden Zusammenhang zwischen Halbflüssen von holomorphen Selbstabbildungen auf einfach zusammenhängenden Gebieten und Halbgruppen, die von Poincaré-Steklov Operatoren erzeugt werden. Mithilfe von Erzeuger von Kompositionshalbgruppen auf Banachräumen von analytischen Funktionen werden insbesondere Dirichlet-zu-Neumann und Dirichlet-zu-Robin Operatoren konstruiert. Dieser Zugang eröffnet einen neuen Ansatz für das Studium partiellen Differentialgleichungen, die mit solchen Operatoren assoziiert sind.We study a surprising connection between semiflows of holomorphic selfmaps of a simply connected domain and semigroups generated by Poincaré-Steklov operators. In particular, by means of generators of semigroups of composition operators on Banach spaces of analytic functions, we construct Dirichlet-to-Neumann and Dirichlet-to-Robin operators. This approach gives new insights to the theory of partial differential equations associated with such operators

Composition Operators from the Hardy Space to the Zygmund-Type Space on the Upper Half-Plane

Here we introduce the nth weighted space on the upper half-plane Π+={z∈ℂ:Im z>0} in the complex plane ℂ. For the case n=2, we call it the Zygmund-type space, and denote it by 𝒵(Π+). The main result of the paper gives some necessary and sufficient conditions for the boundedness of the composition operator Cφf(z)=f(φ(z)) from the Hardy space Hp(Π+) on the upper half-plane, to the Zygmund-type space, where φ is an analytic self-map of the upper half-plane

Composition operators from the weighted . . .

The boundedness of the composition operator from the weighted Bergman space to the recently introduced by the author, the nth weighted space on the unit disc, is characterized. Moreover, the norm of the operator in terms of the inducing function and weights is estimated

On Compactness and Closed-Rangeness of Composition Operators

Let $\phi$ be an analytic self-map of the unit disk $\mathbb{D}:=\{z:\lvert z\rver