Wandering subspace property for homogeneous invariant subspaces

Abstract

For graded Hilbert spaces $H$ and shift-like commuting tuples $T \in B(H)^n$, we show that each homogeneous joint invariant subspace $M$ of $T$ has finite index and is generated by its wandering subspace. Under suitable conditions on the grading $(H_k)_{k\geq 0}$ of $H$ the algebraic direct sum $\tilde{M} = \oplus_{k\geq 0} M \cap H_k$ becomes a finitely generated module over the polynomial ring $\mathbb C[z]$. We show that the wandering subspace $W_T(M)$ of $M$ is contained in $\tilde{M}$ and that each linear basis of $W_T(M)$ forms a minimal set of generators for the $\mathbb C[z]$-module $\tilde{M}$. We describe an algorithm that transforms each set of homogeneous generators of $\tilde{M}$ into a minimal set of generators and can be used in particular to compute minimal sets of generators for homogeneous ideals $I \subset \mathbb C[z]$. We prove that each $\gamma$-graded commuting row contraction $T \in B(H)^n$ admits a finite weak resolution in the sense of Arveson or Douglas and Misra.