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Passive Systems with a Normal Main Operator and Quasi-selfadjoint Systems
Authors:Yury M Arlinski?  Seppo Hassi  Henk S V de Snoo
Institution:(1) Department of Mathematical Analysis, East Ukrainian National University, Kvartal Molodyozhny 20-A, UA-91034 Lugansk, Ukraine;(2) Department of Mathematics and Statistics, University of Vaasa, P.O. Box 700, FIN-65101 Vaasa, Finland;(3) Department of Mathematics and Computing Science, University of Groningen, P.O. Box 407, NL-9700 AK Groningen, Nederland
Abstract:Passive systems $$\tau = \{T,{\mathfrak{M}},{\mathfrak{N}},{\mathfrak{H}}\}$$ with $${\mathfrak{M}}$$ and $${\mathfrak{N}}$$ as an input and output space and $${\mathfrak{H}}$$ as a state space are considered in the case that the main operator on the state space is normal. Basic properties are given and a general unitary similarity result involving some spectral theoretic conditions on the main operator is established. A passive system $$\tau$$ with $${\mathfrak{M}} = {\mathfrak{N}}$$ is said to be quasi-selfadjoint if ran $$(T - T^*) \subset {\mathfrak{N}}$$. The subclass $${\bf S}^{qs}({\mathfrak{N}})$$ of the Schur class $${\bf S}({\mathfrak{N}})$$ is the class formed by all transfer functions of quasi-selfadjoint passive systems. The subclass $${\bf S}^{qs}({\mathfrak{N}})$$ is characterized and minimal passive quasi-selfadjoint realizations are studied. The connection between the transfer function belonging to the subclass $${\bf S}^{qs}({\mathfrak{N}})$$ and the Q-function of T is given. Received: December 16, 2007., Accepted: March 4, 2008.
Keywords:Mathematics Subject Classification (2000)" target="_blank">Mathematics Subject Classification (2000)    Primary 47A45  47A48  47A56  Secondary 93B15  93B28
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