C*-Algebras With and Without $\ll$-Increasing Approximate Units

TL;DR

This paper investigates the existence of $\\ll$-increasing approximate units in C*-algebras, extending classical results to nonseparable cases and constructing new examples that challenge previous assumptions.

Contribution

It extends the theory of approximate units to $\\omega_1$-unital C*-algebras and constructs the first examples of C*-algebras lacking such units, revealing new structural phenomena.

Findings

All $\omega_1$-unital C*-algebras have $\ll$-increasing approximate units.

Constructed examples of C*-algebras with no $\ll$-increasing approximate unit.

Existence of such algebras is independent of ZFC.

Abstract

For elements $a, b$ of a C*-algebra we denote $a=ab$ by $a\ll b$. We show that all $\omega_1$-unital C*-algebras have $\ll$-increasing approximate units, extending a classical result for $\sigma$-unital C*-algebras. We also construct (in ZFC) the first examples of C*-algebras with no $\ll$-increasing approximate unit. One of these examples is a C*-subalgebra of $\mathcal B(\ell_2(\omega_1))$. These examples are, by necessity, not approximately finite dimensional (AF), but some of them are still scattered and so locally finite dimensional (LF) in the sense of Farah and Katsura. It follows that there are scattered C*-algebras which are not AF. We further show that the existence of a C*-subalgebra of $\mathcal B(\ell_2)$ with no $\ll$-increasing approximate unit or the existence of an LF but not AF C*-subalgebra of $\mathcal B(\ell_2)$ are independent of ZFC. Our examples also show that an extension of an AF algebra by an AF algebra does not have to be an AF algebra in the nonseparable case.