Journal of Operator Theory

Volume 62, Issue 2, Fall 2009 pp. 357-370.

The isometric representation theory of a perforated semigroup

Authors: Iain Raeburn (1) and Sean T. Vittadello (2)
Author institution: (1) School of Mathematics and Applied Statistics, University of Wollongong, NSW 2522, Australia
(2) School of Mathematical Sciences, University of Newcastle, Callaghan, NSW 2308, Australia

Summary: We consider the additive subsemigroup

$\Sigma:=\N \setminus \{ 1 \}$ of

$\N$ , and study representations of

$\Sigma$ by isometries on Hilbert space with commuting range projections. Our main theorem says that each such representation is unitarily equivalent to the direct sum of a unitary representation, a multiple of the Toeplitz representation on

$\ell^{2} (\Sigma)$ , and a multiple of a representation by shifts on

$\ell^{2} ( \N )$ . We consider also the

$C^*$ -algebra

$C^*(\Sigma)$ generated by a universal isometric representation with commuting range projections, and use our main theorem to identify the faithful representations of

$C^*(\Sigma)$ and prove a structure theorem for

$C^*(\Sigma)$ .

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