On Dirichlet-type and $n$-isometric shifts in finite rank de~Branges--Rovnyak spaces

Abstract

This paper studies the function spaces $\mathcal{D}(μ)$ by Richter and Aleman, and $\mathcal{D}_{\vecμ}$ by the second author. It is known that the forward shift $M_z$ is bounded and expansive on $\mathcal{D}(μ)$, and therefore $\mathcal{D}(μ)$ coincides with a de~Branges--Rovnyak space $\mathcal{H}[B]$. We show that such a $B$ is rational if and only if $μ$ is finitely atomic, and this happens exactly when the corresponding defect operator has finite rank. We also outline a method for calculating the reproducing kernel of $\mathcal{D}(μ)$ for finitely atomic $μ$. Similarly, we characterize the allowable tuples $\vecμ = (\frac{|dz|}{2π}, μ_1, \ldots, μ_{n-1})$ such that $M_z$ on $\mathcal{D}_{\vecμ}$ is expansive with finite rank defect operator. This investigation provides many interesting examples of normalized allowable tuples $\vecμ$.