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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.