On the coefficient formula for de Branges-Rovnyak norms

Abstract

Let $\mathcal{H}(b)$ be the de Branges-Rovnyak space associated to a non-extreme point $b$ of the unit ball of $H^{\infty }$, and let $\varphi =b/a$, where $a$ is the Pythagorean mate of $b$. It is known that, if $f$ is a function holomorphic on a neighbourhood of the closed unit disk, then it belongs to $\mathcal{H}(b)$, and its norm in $\mathcal{H}(b)$ can be expressed in terms of the Taylor coefficients of $f$ and $\varphi$ via the formula \[ \|f\|_{\mathcal{H}(b)}^2=\sum_{m\ge0}|\hat{f}(m)|^2 +\sum_{m\ge0}\Bigl|\sum_{n\ge0}\overline{\hatϕ(n)}\hat{f}(m+n)\Bigr|^2. \] However, the formula can break down for some other $f\in \mathcal{H}(b)$. In this article we extend the scope of the formula to all $f\in H^2$ for which the right-hand side is finite, provided that either $\varphi \in H^2$ or $\varphi$ is rational. If merely $\varphi \in H^p$ for some $p\in (0,2]$, then the formula still holds provided that, in addition, ${\sum }_{m\ge 0}{m}^{2/p-1}|\hat{f}(m){|}^2<\infty$. We also establish a limit-form of the formula that is valid for all non-extreme $b$ and all $f\in \mathcal{H}(b)$.

18 pages