Weak${}^{\ast }$ polynomial density, diffuseness and proper infinitude

Abstract

The unitary group of a von Neumann algebra on an ${\aleph }_0$-dimensional Hilbert space is polynomially weak${}^{\ast }$-dense (equivalently, $G_{delta}$ dense or residual) in the normal unit ball if and only if the canonical finite atomic quotient of the von Neumann algebra is trivial. The analogous result holds for the usual weak${}^{\ast }$ topology, in which case one can drop the normality constraint. In the latter form, this answers a question of T. Eisner and generalizes the counterpart for full bounded-operator algebras.

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