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Journal of Functional Analysis
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Journal of Functional Analysis
Article . 2006
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A characteristic operator function for the class of n-hypercontractions

A characteristic operator function for the class of \(n\)-hypercontractions
Authors: Olofsson, Anders;

A characteristic operator function for the class of n-hypercontractions

Abstract

Let \(H\) be a complex, not necessarily separable Hilbert space. An operator \(T \in \mathcal{L}(H)\) is called \(n\)-hypercontraction if \(\sum_{k=0}^m (-1)^k\binom{m}{k}T^{*k}T^k\geq 0\) for all \(1\leq m \leq n.\) In particular, \(T\) is a contraction. For \(n>1\), these operators were introduced and characterized by \textit{J.~Agler} [J.~Oper.\ Theory 13, No.~2, 203--217 (1985; Zbl 0593.47022)]. He also showed that \(T\) is a subnormal contraction if and only if \(T\) is an \(n\)-hypercontraction for all \(n\). Let \(\mathcal E\) be another Hilbert space. For \(n\geq 1\), the weighted Bergman space A\(_n(\mathcal E)\) is defined as the Hilbert space of all \(\mathcal E\)-valued functions \(f(z)=\sum_{k\geq 0}a_kz^k\) with \(z\) in the unit disc \(\mathbb D\) and \(a_k \in \mathcal E,\) such that \[ | | f| | ^2=\sum_{k\geq 0}| | a_k| | ^2\frac{1}{\binom{k+n-1}{k}}<\infty. \] (A\(_1(\mathbb C)\) is the Hardy space, whereas A\(_2(\mathbb C)\) is the Bergman space.) The shift operator \(S_n\) on A\(_n(\mathcal E)\) is defined by \(S_n(f)(z)=zf(z)\) with \(z \in \mathbb D\). For an \(n\)-hypercontraction \(T\) of \(C_{0.}\) type, i.\,e., \(T^nx \to 0\) for all vectors \(x\), the author shows that there is an isometry \(V_n\) from \(H\) into A\(_n(\mathcal D_{n,T})\) (with \(\mathcal D_{1,T}\) the usual defect space) such that \(V_nT=S_n^*V_n\). Therefore \(V_n(H)\) is invariant under \(S_n^*\) and its orthogonal complement \(\mathcal I_{n,T}\) is invariant under \(S_n\). The corresponding wandering subspace is \(\mathcal E_{n,T}= \mathcal I_{n,T}\ominus S_n(\mathcal I_{n,T})\). The author proves many results. One of them is the following: A function \(f \in\) A\(_n(\mathcal D_{n,T})\) is in the wandering subspace \(\mathcal E_{n,T}\) if and only if \(f(z)=W_{n,T}(z)x\) for some vector \(x\in \mathcal D_{n,T}^*\), where \(W_{n,T}\) is an operator-valued analytic function in the unit disc whose values are operators between the defect spaces \(\mathcal D_{n,T}^*\) and \(\mathcal D_{n,T}\). The generalized characteristic function \(W_{n,T}\) is the usual Sz.--Nagy--Foias characteristic function when \(n=1\) [\textit{B.~Sz.--Nagy} and \textit{C.~Foias}, ``Harmonic analysis of operators on Hilbert spaces'' (Budapest: Akadémiai Kiadó; Amsterdam-London: North--Holland) (1970; Zbl 0201.45003)].

Keywords

Characteristic operator function, standard weighted Bergman space, Linear operators on special spaces (weighted shifts, operators on sequence spaces, etc.), Wandering subspace, n-Hypercontraction, \(n\)-hypercontraction, reproducing kernel function, Hilbert spaces with reproducing kernels (= (proper) functional Hilbert spaces, including de Branges-Rovnyak and other structured spaces), Hilbert spaces of continuous, differentiable or analytic functions, characteristic operator function, Standard weighted Bergman space, Reproducing kernel function, wandering subspace, Analysis

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
27
Top 10%
Top 10%
Top 10%
hybrid