Journal of Operator Theory

Volume 59, Issue 2, Spring 2008 pp. 359-416.

Coordinate systems and bounded isomorphisms

Authors: Allan P. Donsig (1) and David R. Pitts (2)
Author institution: (1) Department of Mathematics, Univ. of Nebraska-Lincoln, Lincoln, NE, 68588-0130, USA
(2) Department of Mathematics, Univ. of Nebraska-Lincoln, Lincoln, NE, 68588-0130, USA

Summary: For a Banach $\D$-bimodule $\M$ over an abelian unital \cstaralg $\,\D$, we define $\Eigone(\M)$ as the collection of norm-one eigenvectors for the dual action of $\D$ on the Banach space dual $\dual{\M}$. Equip $\Eigone(\M)$ with the weak*-topology. We develop general properties of $\Eigone(\M)$. It is properly viewed as a coordinate system for $\M$ when $\M\subseteq \C$, where $\C$ is a unital \cstaralg \, containing $\D$ as a regular MASA with the extension property; moreover, $\Eigone(\C)$ coincides with Kumjian's twist in the context of \cstardiag s. We identify the \cstar-envelope of a subalgebra $\A$ of a \cstardiag \, when $\D\subseteq\A\subseteq \C$. For triangular subalgebras, each containing the MASA, a bounded isomorphism induces an algebraic isomorphism of the coordinate systems which can be shown to be continuous in certain cases. For subalgebras, each containing the MASA, a bounded isomorphism that maps one MASA to the other MASA induces an isomorphism of the coordinate systems. We show that the weak operator closure of the image of a triangular algebra in an appropriate representation is a CSL algebra and that a bounded isomorphism of triangular algebras extends to an isomorphism of these CSL algebras. We prove that for triangular algebras in our context, any bounded isomorphism is completely bounded. Our methods simplify and extend various known results; for example, isometric isomorphisms of the triangular algebras extend to isometric isomorphisms of the \cstar-envelopes, and the conditional expectation $E:\C\to\D$ is multiplicative when restricted to a triangular subalgebra. Also, we use our methods to prove that the inductive limit of \cstardiag s with regular connecting maps is again a \cstardiag.

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