A Beurling-Blecher-Labuschagne Theorem for noncommutative Hardy spaces associated with semifinite von Neumann algebras

Abstract

In 2008, Blecher and Labuschagne extended Beurling's classical theorem to $H^{\infty }$-invariant subspaces of $L^p(\mathcal{M},\tau )$ for a finite von Neumann algebra $\mathcal{M}$ with a finite, faithful, normal tracial state $\tau$ when $1\le p\le \infty$. In this paper, using Arveson's non-commutative Hardy space $H^{\infty }$ in relation to a von Neumann algebra $\mathcal{M}$ with a semifinite, faithful, normal tracial weight $\tau$, we prove a Beurling-Blecher-Labuschagne theorem for $H^{\infty }$-invariant spaces of $L^p(\mathcal{M},\tau )$ when $0<p\le \infty$. The proof of the main result relies on proofs of density theorems for $L^p(\mathcal{M},\tau )$ and semifinite versions of several other known theorems from the finite case. Using the main result, we are able to completely characterize all $H^{\infty }$-invariant subspaces of $L^p(\mathcal{M}{⋊}_{\alpha }\mathbb{Z},\tau )$, where $\mathcal{M}{⋊}_{\alpha }\mathbb{Z}$ is a crossed product of a semifinite von Neumann algebra $\mathcal{M}$ by the integer group $\mathbb{Z}$ and $H^{\infty }$ is a non-selfadjoint crossed product of $\mathcal{M}$ by ${\mathbb{Z}}^{+}$. As an example, we characterize all $H^{\infty }$-invariant subspaces of the Schatten $p$-class $S^p(\mathcal{H})$, where $H^{\infty }$ is the lower triangular subalgebra of $B(\mathcal{H})$, for each $0<p\le \infty$.