Journal of Operator Theory

Volume 48, Issue 2, Fall 2002 pp. 419-430.

Homotopy of state orbits

Authors: Esteban Andruchow (1), and Alejandro Varela (2)
Author institution: (1) Campus Universitario, Instituto de Ciencias, Universidad Nacional de Gral. Sarmiento, J.M. Gutierrez y Verdi, (1613) Los Polvorines, Argentina
(2) Campus Universitario, Instituto de Ciencias, Universidad Nacional de Gral. Sarmiento, J.M. Gutierrez y Verdi, (1613) Los Polvorines, Argentina

Summary: Let

$M$ be a von Neumann algebra,

$\f$ a faithful normal state and denote by

$M^\f$ the fixed point algebra of the modular group of

$\f$ . Let

$U_M$ and

$U_{M^\f}$ be the unitary groups of

$M$ and

$M^\f$ . In this paper we study the quotient

$\uf =U_M/U_{M^\f}$ endowed with two natural topologies: the one induced by the usual norm of

$M$ (called here {\it usual topology of}

$\uf$ ), and the one induced by the pre-Hilbert

$C^*$ -module norm given by the

$\f$ -invariant conditional expectation

$E_{\f}:M \to M^{\f}$ (called the {\it modular topology}). It is shown that

$\uf$ is simply connected with the usual topology. Both topologies are compared, and it is shown that they coincide if and only if the Jones index of

$E_{\f}$ is finite. The set

$\uf$ can be regarded as a model for the unitary orbit

$\{\f \circ \Ad(u^*): u\in U_M\}$ of

$\f$ , and either with the usual or the modular it can be embedded continuously in the conjugate space

$M^*$ (although not as a topological submanifold).

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