Journal of Operator Theory

Volume 55, Issue 2, Spring 2006 pp. 225-238.

Sectional curvature and commutation of pairs of selfadjoint operators

Authors: E. Andruchow (1) and L. Recht (2)
Author institution: (1) Instituto de Ciencias, Universidad Nacional de General Sarmiento, Los Polvorines, 1613, Argentina
(2) Departamento de Matem\'aticas Puras y Aplicadas, Universidad Sim\'on Bol\'\i var, Caracas, 1080A, Venezuela

Summary: The space

$\g^+$ of postive invertible operators of a

$C^*$ -algebra

$\al$ , with the appropriate Finsler metric, behaves like a (non positively curved) symmetric space. Among the characteristic properties of such spaces, one has that two selfadjoint elements

$x,y\in\al$ (regarded as tangent vectors at

$a\in \g^+$ ) verify that

$\begin{equation*} \|x-y\|_a\leqslant d(\mathrm{exp}_a(x),\mathrm{exp}_a(y)). \end{equation*}$ In this paper we investigate the ocurrence of the equality

$\begin{equation*} \|x-y\|_a=d(\mathrm{exp}_a(x),\mathrm{exp}_a(y)). \end{equation*}$ If

$\al$ has a trace, and the trace is used to measure tangent vectors then, as in the finite dimensional classical setting, this equality is equivalent to the fact that

$x$ and

$y$ commute. In arbitrary

$C^*$ -algebras, when the usual

$C^*$ -norm is used, the equality is equivalent to a weaker condition. We introduce in

$\g^+$ an analogous of the sectional curvature for pairs of selfadjoint operators, and study the vanishing of this invariant.

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