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On imbedding theorems for coinvariant subspaces of the shift operator. II
A. B. Aleksandrov St. Petersburg Department of V. A. Steklov Institute of Mathematics, Russian Academy of Sciences
Abstract:
For every inner function
$\Theta$, we put $\Theta^*(H^2)\overset{\text{def}}=H^2\ominus\Theta H^2$, and $\Theta^*(H^p)\overset{\text{def}}=\operatorname{clos}_{H^p}(H^p\cap\Theta^*(H^2))$ for
$p\ne 2$. Denote $\mathscr C_p(\Theta)=\{\mu\in C(\overline{\mathbb D}):\Theta^*(H^p)\subset L^p(|\mu|)\}$. An inner function
$\Theta$ is said to be one-component if the set
$\{z\in\mathbb D:|\Theta(z)|<\varepsilon\}$ is connected for some
$\varepsilon\in(0,1)$. A series of criteria for that are obtained. For example,
$\Theta$ is one-component if and only if
$\mathscr C_p(\Theta)$ does not depend on
$p\in(0,+\infty)$. Moreover, there is a criterion in terms of the reproducing kernel of
$\Theta^*(H^2)$. The set
$\mathscr C_p(\Theta)$ is described in the case where
$\Theta$ is a Blaschke product of special form. This description implies that the set of all
$p$ such that a given measure
$\mu$ belongs to
$\mathscr C_p(\Theta)$ may have any finite or infinite number of connected component. The following examples of interpolating Blaschke products
$\Theta$ and positive measures
$\mu$ are constructed: (1)
$\Theta^*(H^1)\subset L^1(\mu)$ and
$\Theta^*(H^2)\subset L^2(\mu)$ but
$\Theta^*(H^p)\not\subset L^p(\mu)$ for any
$p\in(1,2)$; (2)
$\Theta^*(H^p)\subset L^p(\mu)$ if and only if
$p=\frac1n$, where
$n$ is a positive integer; (3)
$\Theta^*(H^p)\subset L^p(\mu)$ if and only if
$p\ne\frac1n$, where
$n$ is a positive integer.
UDC:
517.5
Received: 11.05.1999