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Research Papers
$J_{p,m}$-inner dilations of matrix-valued functions that belong to the Carathódory class and admit pseudocontinuation
D. Z. Arov,
N. A. Rozhenko South Ukrainian State Pedagogical University, Odessa
Abstract:
The class
$\ell^{p\times p}$ of matrix-valued functions
$c(z)$ holomorphic in the unit disk
$D=\{{z\in\mathbb{C}:|z|<1}\}$, having order
$p$, and satisfying
$\operatorname{Re}c(z)\ge 0$ in
$D$ is considered, as well as its subclass
$\ell^{p\times p}\Pi$ of matrix-valued functions
$c(z)\in \ell^{p\times p}$ that have a meromorphic pseudocontinuation
$c_-(z)$ to the complement
$D_e=\{z\in\mathbb{C}:1<|z|\le\infty\}$ of the unit disk with bounded Nevanlinna characteristic in
$D_e$.
For matrix-valued functions
$c(z)$ of class
$\ell^{p\times p}\Pi$ a representation as a block of a certain
$J_{p,m}$-inner matrix-valued function
$\theta(z)$ is obtained. The latter function has a special structure and is called the
$J_{p,m}$-inner dilation of
$c(z)$. The description of all such representations is given.
In addition, the following special
$J_{p,m}$-inner dilations are considered and described: minimal, optimal,
$*$-optimal, minimal and optimal, minimal and
$*$-optimal. Also,
$J_{p,m}$-inner dilations with additional properties are treated: real, symmetric, rational, or any combination of them under the corresponding restrictions on the matrix-valued function
$c(z)$. The results extend to the case where the open upper half-plane
$\mathbb{C}_+$ is considered instead of the unit disk
$D$. For entire matrix-valued functions
$c(z)$ with
$\operatorname{Re}c(z)\ge 0$ in
$\mathbb{C_+}$ and with Nevanlinna characteristic in
$\mathbb{C}_-$, the
$J_{p,m}$-inner dilations in
$\mathbb{C}_+$ that are entire matrix-valued functions are also described.
Keywords:
Holomorphic matrix-valued functions, dilations, pseudocontinuation.
MSC: 20G35 Received: 09.11.2006