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POSITIVE DEFINITE FUNCTIONS AND SEBESTYÉN'S OPERATOR MOMENT PROBLEM

Published online by Cambridge University Press:  29 November 2005

ZOLTÁN SEBESTYÉN
Affiliation:
Department of Applied Analysis, Loránd Eötvös University, 1117 Budapest, Pázmény Péter sétány 1/C, Hungary e-mail: sebesty@cs.elte.hu
DAN POPOVICI
Affiliation:
Department of Mathematics, University of the West, 300223 Timişoara, Bd. V. Pârvan 4, Romania e-mail: popovici@math.uvt.ro
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Abstract

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Given a sequence $\{A_n\}_{n\in\mathbb{Z}_+}$ of bounded linear operators between complex Hilbert spaces $\mathcal{H}$ and $\mathcal{K}$ we characterize the existence of a contraction (resp. isometry, unitary operator, shift) $T$ on $\mathcal{K}$ such that \[A_n=T^nA_0,\quad n\in\mathbb{Z}_+.\] Such moment problems are motivated by their connection with the dilatability of positive operator measures having applications in the theory of stochastic processes.

The solutions, based on the fact that a certain operator function attached to $T$ is positive definite on $\mathbb{Z}$, extend the ones given by Sebestyén in [18], [19] or, recently, by Jabłoński and Stochel in [8]. Some applications, containing new characterizations for isometric, unitary operators, orthogonal projections or commuting pairs having regular dilation, conclude the paper.

Type
Research Article
Copyright
2005 Glasgow Mathematical Journal Trust