A Murray-von Neumann type classification of $C^{\ast }$-algebras

Abstract

We define type $\mathfrak{A}$, type $\mathfrak{B}$, type $\mathfrak{C}$ as well as C*-semi-finite C*-algebras. It is shown that a von Neumann algebra is a type $\mathfrak{A}$, type $\mathfrak{B}$, type $\mathfrak{C}$ or C*-semi-finite C*-algebra if and only if it is, respectively, a type I, type II, type III or semi-finite von Neumann algebra. Any type I C*-algebra is of type $\mathfrak{A}$ (actually, type $\mathfrak{A}$ coincides with the discreteness as defined by Peligrad and Zsido), and any type II C*-algebra (as defined by Cuntz and Pedersen) is of type $\mathfrak{B}$. Moreover, any type $\mathfrak{C}$ C*-algebra is of type III (in the sense of Cuntz and Pedersen). Furthermore, any purely infinite C*-algebra (in the sense of Kirchberg and Rordam) with real rank zero is of type $\mathfrak{C}$, and any separable purely infinite C*-algebra with stable rank one is also of type $\mathfrak{C}$. We also prove that type $\mathfrak{A}$, type $\mathfrak{B}$, type $\mathfrak{C}$ and C*-semi-finiteness are stable under taking hereditary C*-subalgebras, multiplier algebras and strong Morita equivalence. Furthermore, any C*-algebra $A$ contains a largest type $\mathfrak{A}$ closed ideal $J_{\mathfrak{A}}$, a largest type $\mathfrak{B}$ closed ideal $J_{\mathfrak{B}}$, a largest type $\mathfrak{C}$ closed ideal $J_{\mathfrak{C}}$ as well as a largest C*-semi-finite closed ideal $J_{\mathfrak{sf}}$. Among them, we have $J_{\mathfrak{A}}+J_{\mathfrak{B}}$ being an essential ideal of $J_{\mathfrak{sf}}$, and $J_{\mathfrak{A}}+J_{\mathfrak{B}}+J_{\mathfrak{C}}$ being an essential ideal of $A$. On the other hand, $A/J_{\mathfrak{C}}$ is always C*-semi-finite, and if $A$ is C*-semi-finite, then $A/J_{\mathfrak{B}}$ is of type $\mathfrak{A}$.

28 pages. The appendix in the original version of this paper was removed, and was then expanded to another paper (in Chinese). The current version of this paper is appeared in the proceedings of the conference: "Operator Semigroups Meet Complex Analysis, Harmonic Analysis and Mathematical Physics", took place in Herrnhut, Germany, in honor of Charles Batty for his 60th birthday