On Murray-von Neumann algebras—I: topological, order-theoretic and analytical aspects

Abstract

For a countably decomposable finite von Neumann algebra ${\mathscr {R}}$, we show that any choice of a faithful normal tracial state on ${\mathscr {R}}$ engenders the same measure topology on ${\mathscr {R}}$ in the sense of Nelson (J Funct Anal 15:103–116, 1974). Consequently it is justified to speak of ‘the’ measure topology of ${\mathscr {R}}$. Having made this observation, we extend the notion of measure topology to general finite von Neumann algebras and denominate it the ${\mathfrak {m}}$-topology. We note that the procedure of ${\mathfrak {m}}$-completion yields Murray-von Neumann algebras in a functorial manner and provides them with an intrinsic description as unital ordered complex topological $*$-algebras. This enables the study of abstract Murray-von Neumann algebras avoiding reference to a Hilbert space. Furthermore, it makes apparent the appropriate notion of Murray-von Neumann subalgebras, and the intrinsic nature of the spectrum and point spectrum of elements, independent of their ambient Murray-von Neumann algebra. In this context, we show the well-definedness of the Borel function calculus for normal elements and use it along with approximation techniques in the ${\mathfrak {m}}$-topology to transfer many standard operator inequalities involving bounded self-adjoint operators to the setting of (unbounded) self-adjoint operators in Murray-von Neumann algebras. On the algebraic side, Murray-von Neumann algebras have been described as the Ore localization of finite von Neumann algebras with respect to their corresponding multiplicative subset of non-zero-divisors. Our discussion reveals that, in addition, there are fundamental topological, order-theoretic and analytical facets to their description.