Covariant representations of subproduct systems: Invariant subspaces and curvature

Abstract: Let $X=(X(n)){n \in \mathbb{Z+}}$ be a standard subproduct system of $C^*$-correspondences over a $C^*$-algebra $\mathcal M.$ Assume $T=(T_n){n \in \mathbb{Z+}}$ to be a pure completely contractive, covariant representation of $X$ on a Hilbert space $\mathcal H,$ and $\mathcal S$ to be a non-trivial closed subspace of $\mathcal H.$ Then $\mathcal{S}$ is invariant for $T$ if and only if there exist a Hilbert space $\mathcal{D},$ a representation $\pi$ of $\mathcal M$ on $\mathcal D,$ and a partial isometry $\Pi: \mathcal{F}X\bigotimes{\pi}\mathcal{D}\to \mathcal{H} $ such that $$\Pi (S_n(\zeta)\otimes I_{\mathcal{D}})=T_n(\zeta)\Pi~\mbox{whenever}~\zeta\in X(n), ~n\in \mathbb{Z_+},~\mbox{and}$$ $\mathcal S$ is the range of $\Pi,$ or equivalently, $P_{\mathcal S}=\Pi\Pi^*.$ This result leads us to many important consequences including Beurling type theorem and other general observations on wandering subspaces. We extend the notion of curvature for completely contractive, covariant representations and analyze it in terms of the above results.