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Published online by Cambridge University Press: 20 November 2018
For monotone complete ${{C}^{*}}$ -algebras
$A\subset B$ with
$A$ contained in
$B$ as a monotone closed
${{C}^{*}}$ -subalgebra, the relation
$X=AsA$ gives a bijection between the set of all monotone closed linear subspaces
$X$ of
$B$ such that
$AX+XA\subset X$ and
$X{{X}^{*}}+{{X}^{*}}X\subset A$ and a set of certain partial isometries
$s$ in the “normalizer” of
$A$ in
$B$ , and similarly for the map
$s\mapsto \text{Ad }s$ between the latter set and a set of certain “partial
$*$ -automorphisms” of
$A$ . We introduce natural inverse semigroup structures in the set of such
$X$ 's and the set of partial
$*$ -automorphisms of
$A$ , modulo a certain relation, so that the composition of these maps induces an inverse semigroup homomorphism between them. For a large enough
$B$ the homomorphism becomes surjective and all the partial
$*$ -automorphisms of
$A$ are realized via partial isometries in
$B$ . In particular, the inverse semigroup associated with a type
$\text{I}{{\text{I}}_{1}}$ von Neumann factor, modulo the outer automorphism group, can be viewed as the fundamental group of the factor. We also consider the
${{C}^{*}}$ -algebra version of these results.