Abstract view
Admissible Majorants for Model Subspaces of $H^2$, Part II: Fast Winding of the Generating Inner Function


Published:20031201
Printed: Dec 2003
Victor Havin
Javad Mashreghi
Abstract
This paper is a continuation of Part I [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 $f(x)\leq\omega(x)$ 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 $\Theta(x)$ winds up fast as $x$ grows from $\infty$
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 $(0,\infty)$ 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 $\exp(i\sigma z)$, $\sigma>0$. Some oscillating $\omega$'s in
Adm $B$ are also described. Our theme is related to the
BeurlingMalliavin multiplier theorem devoted to Adm $\exp(i\sigma z)$,
$\sigma>0$, and to de Branges' space $\mathcal{H}(E)$.
Keywords: 
Hardy space, inner function, shift operator, model, subspace, Hilbert transform, admissible majorant
Hardy space, inner function, shift operator, model, subspace, Hilbert transform, admissible majorant
