Type III von Neumann Algebras associated with $\O_\theta$

Abstract

Let $\Fth$ be a 2 graph generated by $m$ blue edges and $n$ red edges, and $\omega$ be the distinguished faithful state associated with its graph C*-algebra $\O_\theta$. In this paper, we characterize the factorness of the von Neumann algebra $\pi_\omega(\O_\theta)"$ induced from the GNS representation of $\omega$ under a certain condition. Moreover, when $\pi_\omega(\O_\theta)"$ is a factor, then it is of type III$_{m^{-\frac{1}{b}}}$ (or III$_{n^{-\frac{1}{a}}}$) if $\frac{\ln m}{\ln n}\in\bQ$, where $a,b\in\bN$ with $\gcd(a,b)=1$ satisfy $m^a=n^b$, and of type III$_1$ if $\frac{\ln m}{\ln n}\not\in\bQ$. In the case of $\theta$ being the identity permutation, our condition turns out to be redundant. On the way to our main results, we also obtain the structure of the fixed point algebra $\O_\theta^\sigma$ of the modular action $\sigma$ from $\omega$. This could be useful in proving the redundancy of our extra condition.