Von Neumann Algebras and Extensions of Inverse Semigroups

In the 1970s, Feldman and Moore classified separably acting von Neumann algebras containing Cartan MASAs using measured equivalence relations and 2-cocycles on such equivalence relations. In this paper, we give a new classification in terms of extensions of inverse semigroups. Our approach is more algebraic in character and less point-based than that of Feldman-Moore. As an application, we give a restatement of the spectral theorem for bimodules in terms of subsets of inverse semigroups. We also show how our viewpoint leads naturally to a description of maximal subdiagonal algebras.
Comment: Applications added: i) a reformulation of the spectral theorem for Bures-closed bimodules and ii) a description of maximal subdiagonal algebras. 38 pages