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JOURNALS // Zapiski Nauchnykh Seminarov POMI // Archive

Zap. Nauchn. Sem. POMI, 1999 Volume 262, Pages 5–48 (Mi znsl1104)

This article is cited in 34 papers

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


 English version:
Journal of Mathematical Sciences (New York), 2002, 110:5, 2907–2929

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