De Branges-Rovnyak spaces and local Dirichlet spaces of higher order

Abstract

We discuss de Branges-Rovnyak spaces $\mathcal{H}(b)$ generated by nonextreme and rational functions $b$ and local Dirichlet spaces of order $m$ introduced in [6]. In [6] the authors characterized nonextreme $b$ for which the operator $Y=S{|}_{\mathcal{H}(b)}$, the restriction of the shift operator $S$ on $H^2$ to $\mathcal{H}(b)$, is a strict $2m$-isometry and proved that such spaces $\mathcal{H}(b)$ are equal to local Dirichlet spaces of order $m$. Here we give a characterization of local Dirichlet spaces of order $m$ in terms of the $m$-th derivatives that is a generalization of a known result on local Dirichlet spaces. We also find explicit formulas for $b$ in the case when $\mathcal{H}(b)$ coincides with local Dirichlet space of order $m$ with equality of norms. Finally, we prove a property of wandering vectors of $Y$ analogous to the property of wandering vectors of the restriction of $S$ to harmonically weighted Dirichlet spaces obtained by D. Sarason in [11].

11 pages