A counterexample concerning the coefficient formula for de Branges-Rovnyak norms
Thomas Ransford
所有作者
Thomas Ransford
文献摘要
Let H(b) be the de Branges-Rovnyak space associated to a non-extreme point b of the unit ball of H∞, and let φ=b/a, where a is the Pythagorean mate of b. In an earlier article, the author showed that, if φ∈H2, then f∈H(b)⟺(f∈H2 and m≥0∑n≥0∑φ^(n)f^(m+n)2<∞), in which case we have ∥f∥H(b)2=m≥0∑∣f^(m)∣2+m≥0∑n≥0∑φ^(n)f^(m+n)2. It was also shown that there exists a space H(b) for which φ∈∩p<2Hp, yet f∈H(b)⇒m≥0∑n≥0∑φ^(n)f^(m+n)2<∞. Here we complete this circle of ideas by showing that there exists a space H(b) for which φ∈∩p<1Hp, yet (f∈H2 and m≥0∑n≥0∑φ^(n)f^(m+n)2<∞)⇒f∈H(b).