Doubly commuting invariant and co-invariant subspaces

Abstract

Given complex separable Hilbert spaces $\mathcal{H}_i$ and bounded operators $T_i$ on $\mathcal{H}_i$, for $i=1,\ldots,n$, we consider the tensor product space $\mathfrak{H}:=\mathcal{H}_1\otimes\cdots\otimes\mathcal{H}_n$ and a tuple $(\tilde{T}_1,\ldots,\tilde{T}_n)$ on $\mathfrak{H}$ where \[\tilde{T}_i=I_{\mathcal{H}_1}\otimes\cdots\otimes I_{\mathcal{H}_{i-1}}\otimes T_i\otimes I_{\mathcal{H}_{i+1}}\otimes\cdots\otimes I_{\mathcal{H}_n}.\] A closed jointly $(\tilde{T}_1,\ldots,\tilde{T}_n)$-invariant subspace $\mathfrak{M}$ of $\mathfrak{H}$ is said to be doubly commuting if the tuple $(\tilde{T}_1|_{\mathfrak{M}},\ldots,\tilde{T}_n|_{\mathfrak{M}})$ is doubly commuting. In this paper, we show that if the hereditary polynomials in each $T_i$ are dense in $\mathcal{B}(\mathcal{H}_i)$ with respect to the weak operator topology for $i=1,\ldots,n$, then doubly commuting jointly $(\tilde{T}_1,\ldots,\tilde{T}_n)$-invariant subspaces $\mathfrak{M}$ of $\mathfrak{H}$ are characterized in the form \[\mathcal{M}_1\otimes\cdots\otimes\mathcal{M}_n,\] where $\mathcal{M}_i\subseteq\mathcal{H}_i$ is $T_i$-invariant, for each $i=1,\ldots,n$. As applications, we provide a unified framework for understanding and characterizing doubly commuting quotient modules of weighted Bergman spaces over the open unit polydisc, doubly commuting quotient modules of tensor products of complete Nevanlinna-Pick spaces, and doubly commuting invariant and co-invariant subspaces of certain operators acting on tensor products of de Branges-Rovnyak spaces $H(b)$ associated with both extreme and nonextreme points $b$. This recovers, through a different approach, the known characterizations of doubly commuting quotient modules of the Hardy space over the open unit polydisc and doubly commuting submodules of Jordan blocks of Hardy space over the open unit polydisc.