Dilations of some VH-spaces operator valued invariant kernels

Cataloged from PDF version of article.We investigate VH-spaces (Vector Hilbert spaces, or Loynes spaces) operator valued Hermitian kernels that are invariant under actions of *-semigroups from the point of view of generation of *-representations, linearizations (Kolmogorov decompositions), and reproducing kernel spaces. We obtain a general dilation theorem in both Kolmogorov and reproducing kernel space representations, that unifies many dilation results, in particular B. Sz.-Nagy's and Stinesprings' dilation type theorems. © 2012 Springer Basel

Embeddings, operator ranges, and Dirac operators

Cataloged from PDF version of article.Motivated by energy space representation of Dirac operators, in the sense of K. Friedrichs, we recently introduced the notion of closely embedded Kreǐn spaces. These spaces are associated to unbounded selfadjoint operators that play the role of kernel operators, in the sense of L. Schwartz, and they are special representations of induced Kreǐn spaces. In this article we present a canonical representation of closely embedded Kreǐn spaces in terms of a generalization of the notion of operator range and obtain a characterization of uniqueness. When applied to Dirac operators, the results differ according to a mass or a massless particle in a dramatic way: in the case of a particle with a nontrivial mass we obtain a dual of a Sobolev type space and we have uniqueness, while in the case of a massless particle we obtain a dual of a homogenous Sobolev type space and we lose uniqueness. © 2010 Elsevier Inc

Pauli algebraic forms of normal and non-normal operators

Cataloged from PDF version of article.A unified treatment of the Pauli algebraic forms of the linear operators defined on a unitary linear space of two dimensions over the field of complex numbers C1 is given. The Pauli expansions of the normal and nonnormal operators, unitary and Hermitian operators, orthogonal projectors, and symmetries are deduced in this frame. A geometrical interpretation of these Pauli algebraical results is given. With each operator, one can associate a generally complex vector, its Pauli axis. This is a natural generalization of the well-known Poincaré axis of some normal operators. A geometric criterion of distinction between the normal and nonnormal operators by means of this vector is established. The results are exemplified by the Pauli representations of the normal and nonnormal operators corresponding to some widespread composite polarization devices. © 2006 Optical Society of America

Closed Embeddings of Hilbert Spaces

Cataloged from PDF version of article.Motivated by questions related to embeddings of homogeneous Sobolev spaces and to comparison of function spaces and operator ranges, we introduce the notion of closely embedded Hilbert spaces as an extension of that of continuous embedding of Hilbert spaces. We show that this notion is a special case of that of Hilbert spaces induced by unbounded positive selfadjoint operators that corresponds to kernel operators in the sense of L. Schwartz. Certain canonical representations and characterizations of uniqueness of closed embeddings are obtained. We exemplify these constructions by closed, but not continuous, embeddings of Hilbert spaces of holomorphic functions. An application to the closed embedding of a homogeneous Sobolev space on Rn in L2(Rn), based on the singular integral operator associated to the Riesz potential, and a comparison to the case of the singular integral operator associated to the Bessel potential are also presented. As a second application we show that a closed embedding of two operator ranges corresponds to absolute continuity, in the sense of T. Ando, of the corresponding kernel operators. © 2010 Elsevier In

Closely embedded Krein spaces and applications to Dirac operators

Cataloged from PDF version of article.Motivated by energy space representation of Dirac operators, in the sense of K. Friedrichs, we recently introduced the notion of closely embedded Krein spaces. These spaces are associated to unbounded selfadjoint operators that play the role of kernel operators, in the sense of L Schwartz, and they are special representations of induced Krein spaces. In this article we present a canonical representation of closely embedded Krein spaces in terms of a generalization of the notion of operator range and obtain a characterization of uniqueness. When applied to Dirac operators, the results differ according to a mass or a massless particle in a dramatic way: in the case of a particle with a nontrivial mass we obtain a dual of a Sobolev type space and we have uniqueness, while in the case of a massless particle we obtain a dual of a homogenous Sobolev type space and we lose uniqueness. (C) 2010 Elsevier Inc. All rights reserved

Triplets of Closely Embedded Dirichlet type spaces on the Unit Polydisc

Cataloged from PDF version of article.We propose a general concept of triplet of Hilbert spaces with closed embeddings, instead of continuous ones, and we show how rather general weighted L2 spaces yield this kind of generalized triplets of Hilbert spaces for which the underlying spaces and operators can be explicitly calculated. Then we show that generalized triplets of Hilbert spaces with closed embeddings can be naturally associated to any pair of Dirichlet type spaces Dα(DN) of holomorphic functions on the unit polydisc DN and we explicitly calculate the associated operators in terms of reproducing kernels and radial derivative operators. We also point out a rigging of the Hardy space H2(DN) through a scale of Dirichlet type spaces and Bergman type spaces. © 2012 Springer Basel

When are the products of normal operators normal?

Given two normal operators A and B on a Hilbert space it is known that, in general, AB is not normal. The question on characterizing those pairs of normal operators for which their products are normal has been solved for finite dimensional spaces by F.R. Gantmaher and M. G. Krein in 1930, and for compact normal operators by N.A. Wiegmann in 1949. Actually, in the afore mentioned cases, the normality of AB is equivalent with that of BA, and a more general result of F. Kittaneh implies that it is sufficient that AB be normal and compact to obtain that BA is the same. On the other hand, I. Kaplansky had shown that it may be possible that AB is normal while BA is not. When no compactness assumption is made, but both of AB and BA are supposed to be normal, the Gantmaher-KreinWiegmann Theorem can be extended by means of the spectral theory of normal operators in the von Neumann's direct integral representation

Operator models for hilbert locally c*-modules

We single out the concept of concrete Hilbert module over a locally C*-algebra by means of locally bounded operators on certain strictly inductive limits of Hilbert spaces. Using this concept, we construct an operator model for all Hilbert locally C*-modules and, as an application, we obtain a direct construction of the exterior tensor product of Hilbert locally C*-modules. These are obtained as consequences of a general dilation theorem for positive semidefinite kernels invariant under an action of a ∗-semigroup with values locally bounded operators. As a by-product, we obtain two Stinespring type theorems for completely positive maps on locally C*-algebras and with values locally bounded operators. © 2017, Element D.O.O. All rights reserved

On Two Equivalent Dilation Theorems in VH-Spaces

We prove that a generalized version, essentially obtained by R. M. Loynes, of the B. Sz.-Nagy's Dilation Theorem for B*(H)-valued (here H is a VH-space in the sense of Loynes) positive semidefinite maps on *-semigroups is equivalent with a generalized version of the W. F. Stinespring's Dilation Theorem for B*(H)-valued completely positive linear maps on B*-algebras. This equivalence result is a generalization of a theorem of F. H. Szafraniec, originally proved for the case of operator valued maps (that is, when H is a Hilbert space). © 2011 Springer Basel AG