Maximal Haagerup subalgebras in $L(\mathbb{Z}^2\rtimes SL_2(\mathbb{Z}))

• Main Author: Jiang, Yongle
• Format: Journal Article
• Language: English
• Published: 02.03.2020
• Subjects: Mathematics - Operator Algebras
• DOI: 10.48550/arxiv.2003.00687

J. Operator Theory 86 (2021), no. 1, 203--230 We prove that $L(SL_2(\textbf{k}))$ is a maximal Haagerup von Neumann subalgebra in $L(\textbf{k}^2\rtimes SL_2(\textbf{k}))$ for $\textbf{k}=\mathbb{Q}$. Then we show how to modify the proof to handle $\textbf{k}=\mathbb{Z}$. The key step for the proof is a complete description of all intermediate von Neumann subalgebras between $L(SL_2(\textbf{k}))$ and $L^{\infty}(Y)\rtimes SL_2(\textbf{k})$, where $SL_2(\textbf{k})\curvearrowright Y$ denotes the quotient of the algebraic action $SL_2(\textbf{k})\curvearrowright \widehat{\textbf{k}^2}$ by modding out the relation $\phi\sim \phi'$, where $\phi$, $\phi'\in \widehat{\textbf{k}^2}$ and $\phi'(x, y):=\phi(-x, -y)$ for all $(x, y)\in \textbf{k}^2$. As a by-product, we show $L(PSL_2(\mathbb{Q}))$ is a maximal von Neumann subalgebra in $L^{\infty}(Y)\rtimes PSL_2(\mathbb{Q})$; in particular, $PSL_2(\mathbb{Q})\curvearrowright Y$ is a prime action, i.e. it admits no non-trivial quotient actions.