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Presented in this volume are a number of new results concerning the extension theory and spectral theory of unbounded operators using the recent notions of boundary triplets and boundary relations. This approach relies on linear single-valued and multi-valued maps, isometric in a Krein space sense, and offers a basic framework for recent developments in system theory. Central to the theory are analytic tools such as Weyl functions, including Titchmarsh-Weyl m-functions and Dirichlet-to-Neumann maps. A wide range of topics is considered in this context from the abstract to the applied, including boundary value problems for ordinary and partial differential equations; infinite-dimensional perturbations; local point-interactions; boundary and passive control state/signal systems; extension theory of accretive, sectorial and symmetric operators; and Calkin's abstract boundary conditions. This accessible treatment of recent developments, written by leading researchers, will appeal to a broad range of researchers, students and professionals.
This volume, which is dedicated to Heinz Langer, includes biographical material and carefully selected papers. Heinz Langer has made fundamental contributions to operator theory. In particular, he has studied the domains of operator pencils and nonlinear eigenvalue problems, the theory of indefinite inner product spaces, operator theory in Pontryagin and Krein spaces, and applications to mathematical physics. His works include studies on and applications of Schur analysis in the indefinite setting, where the factorization theorems put forward by Krein and Langer for generalized Schur functions, and by Dijksma-Langer-Luger-Shondin, play a key role. The contributions in this volume reflect Heinz Langer’s chief research interests and will appeal to a broad readership whose work involves operator theory.
The focus program on Analytic Function Spaces and their Applications took place at Fields Institute from July 1st to December 31st, 2021. Hilbert spaces of analytic functions form one of the pillars of complex analysis. These spaces have a rich structure and for more than a century have been studied by many prominent mathematicians. They also have several essential applications in other fields of mathematics and engineering, e.g., robust control engineering, signal and image processing, and theory of communication. The most important Hilbert space of analytic functions is the Hardy class H2. However, its close cousins, e.g. the Bergman space A2, the Dirichlet space D, the model subspaces Kt,...
Generalized Schur functions are scalar- or operator-valued holomorphic functions such that certain associated kernels have a finite number of negative squares. This book develops the realization theory of such functions as characteristic functions of coisometric, isometric, and unitary colligations whose state spaces are reproducing kernel Pontryagin spaces. This provides a modern system theory setting for the relationship between invariant subspaces and factorization, operator models, Krein-Langer factorizations, and other topics. The book is intended for students and researchers in mathematics and engineering. An introductory chapter supplies background material, including reproducing kernel Pontryagin spaces, complementary spaces in the sense of de Branges, and a key result on defining operators as closures of linear relations. The presentation is self-contained and streamlined so that the indefinite case is handled completely parallel to the definite case.
Expository articles describing the role Hardy spaces, Bergman spaces, Dirichlet spaces, and Hankel and Toeplitz operators play in modern analysis.
It is well known that two hermitian n x n matrices K, H, where H is positive definite, H> 0, can be simultaneously diagonalized. The key to the proof is to consider C[superscript]n, where C is the complex number field, as a Hilbert space [Fraktur capital]H [subscript]H with the inner product given by (f, g) = g*Hf, where f, g [lowercase Greek]Epsilon C[superscript]n, considered as a space of column vectors. Then the operator A = H−1K is selfadjoint in [Fraktur capital]H [subscript]H, and the spectral theorem readily yields the result. Of course such A, when K is not hermitian, can also be investigated in [Fraktur capital]H [subscript]H. We consider a similar problem where K, H are replaced by a pair of ordinary differential expressions L and M, where M> 0 in some sense. Two difficulties arise: (1) there are many natural choices for a selfadjoint H> 0 generated by M, and hence many choices for [Fraktur capital]H [subscript]H, and (2), once a choice for H has been made, there are many choices for the analogue of A. In our work we consider all possible choices for H> 0 and the analogue of A.
This volume consists of a long monographic paper by J. Hoffmann-Jorgensen and a number of shorter research papers and survey articles covering different aspects of functional analysis and its application to probability theory and differential equations.
"In most of the industrialized Western world, the birth process has been almost completely removed from the domain of the woman and the family into the realm of technocratic specialists. To imagine that there exists an industrialized country, the Netherlands, with all the resources of modern medicine, of pharmacology and surgery, where women and care providers actively espouse a noninterventionist stance in childbirth, has always been one of the great puzzles, paradoxes, and revelations in our field. This book traces this most anomalous phenomenon."--Back cover.