A counterexample concerning the coefficient formula for de Branges-Rovnyak norms

Thomas Ransford

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 $b$…