
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)].
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
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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