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Admissible Majorants for Model Subspaces of H2, Part II: Fast Winding of the Generating Inner Function

Published online by Cambridge University Press:  20 November 2018

Victor Havin
Affiliation:
Department of Mathematics and Statistics, McGill University, Montreal, Quebec, H3A 2K6 e-mail: havin@havin.usr.pu.ru Department of Mathematics and Mechanics, St. Petersburg State University, Russia 198904
Javad Mashreghi
Affiliation:
Département de mathématiques et de statistique, Université Laval, Laval, Québec, G1K 7P4 e-mail: Javad.Mashreghi@mat.ulaval.ca
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Abstract

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This paper is a continuation of $[6]$. We consider the model subspaces ${{K}_{\Theta }}={{H}^{2}}\ominus \Theta {{H}^{2}}$ of the Hardy space ${{H}^{2}}$ generated by an inner function $\Theta $ in the upper half plane. Our main object is the class of admissible majorants for ${{K}_{\Theta }}$, denoted by Adm $\Theta $ and consisting of all functions $\omega $ defined on $\mathbb{R}$ such that there exists an $f\ne 0,f\in {{K}_{\Theta }}$ satisfying $|f\left( x \right)|\,\le \,\omega \left( x \right)$ almost everywhere on $\mathbb{R}$. Firstly, using some simple Hilbert transform techniques, we obtain a general multiplier theorem applicable to any ${{K}_{\Theta }}$ generated by a meromorphic inner function. In contrast with $[6]$, we consider the generating functions $\Theta $ such that the unit vector $\Theta \left( x \right)$ winds up fast as $x$ grows from $-\infty \,\text{to}\,\infty $. In particular, we consider $\Theta \,=\,B$ where $B$ is a Blaschke product with “horizontal” zeros, i.e., almost uniformly distributed in a strip parallel to and separated from $\mathbb{R}$. It is shown, among other things, that for any such $B$, any even $\omega $ decreasing on $\left( 0,\,\infty \right)$ with a finite logarithmic integral is in Adm $B$ (unlike the “vertical” case treated in $[6]$), thus generalizing (with a new proof) a classical result related to Adm $\exp \left( i\sigma z \right),\,\sigma \,>\,0$. Some oscillating $\omega $'s in Adm $B$ are also described. Our theme is related to the Beurling-Malliavin multiplier theorem devoted to Adm $\exp \left( i\sigma z \right),\,\sigma \,>\,0$, and to de Branges’ space $H\left( E \right)$.

Type
Research Article
Copyright
Copyright © Canadian Mathematical Society 2003

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