Invariant subspaces for certain tuples of operators with applications to reproducing kernel correspondences

Abstract

The techniques developed by Popescu, Muhly-Solel and Good for the study of algebras generated by weighted shifts are applied to generalize results of Sarkar and of Bhattacharjee-Eschmeier-Keshari-Sarkar concerning dilations and invariant subspaces for commuting tuples of operators. In that paper the authors prove Beurling-Lax-Halmos type results for commuting tuples $T=(T_1,\ldots,T_d)$ operators that are contractive and pure; that is $\Phi_T(I)\leq I$ and $\Phi_T^n(I)\searrow 0$ where $$\Phi_T(a)=\Sigma_i T_iaT_i^*.$$ Here we generalize some of their results to commuting tuples $T$ satisfying similar conditions but for $$\Phi_T(a)=\Sigma_{\alpha \in \mathbb{F}^+_d} x_{|\alpha|}T_{\alpha}aT_{\alpha}^*$$ where $\{x_k\}$ is a sequence of non negative numbers satisfying some natural conditions (where $T_{\alpha}=T_{\alpha(1)}\cdots T_{\alpha(k)}$ for $k=|\alpha|$). In fact, we deal with a more general situation where each $x_k$ is replaced by a $d^k\times d^k$ matrix. We also apply these results to subspaces of certain reproducing kernel correspondences $E_K$ (associated with maps-valued kernels $K$) that are invariant under the multipliers given by the coordinate functions.