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Taurida Journal of Computer Science Theory and Mathematics, 2017 Issue 1, Pages 7–16 (Mi tvim10)

Minimality of selfadjoint dilation of operator knot of dissipative operator

A. V. Bidanets, Yu. L. Kudryashov

Crimea Federal University, Simferopol

Abstract: Let $A$ is dissipative densely defined operator in the space $\mathfrak{H}$ and $-i \in \rho\left(A\right)$.
Let denote $R = {\left(A+iI\right)}^{-1}$ and consider the defect operators
\begin{gather*} B = iR - i{R}^{*} -2{R}^{*}R,\\ \widetilde{B} = iR - i{R}^{*} - 2R{R}^{*},\\ T = I - 2iR. \end{gather*}
A set of linear bounded operators acting from an entire Hilbert space ${H}_{1}$ into a Hilbert space ${H}_{2}$ will be denoted by $L\left({H}_{1}, {H}_{2}\right)$.
Definition. The assembly of Hilbert spaces $\mathfrak{H}$, ${E}_{-}$ è ${E}_{+}$ and operators $A: \mathfrak{H} \rightarrow \mathfrak{H}, \Phi \in L\left({E}_{-}, \mathfrak{H}\right), \Psi \in L\left(\mathfrak{H}, {E}_{+}\right), K \in L\left({E}_{-}, {E}_{+}\right)$ is called the operator knot, which has been introduced in work of U. L. Kudryashov «Selfadjoint dilation of operator knot of dissipative operator» in «Dynamic systems», 3(31), ¹1-2, 2013, p. 45-48${}^{\left[1\right]}$.
$Q = \left(A, \Phi, K, \Psi, \mathfrak{H}, {E}_{-}, {E}_{+}\right)$, if the following relations hold:
\begin{gather*} B = {\Psi}^{*}\Psi;\\ \widetilde{B} = {\Phi}{\Phi}^{*};\\ {T}^{*}\Phi + {\Psi}^{*}K = 0;\\ T{\Psi}^{*}+{\Phi}{K}^{*} = 0;\\ 2{\Phi}^{*}{\Phi}+{K}^{*}K = I;\\ 2{\Psi}{\Psi}^{*} + K{K}^{*} = I. \end{gather*}

Selfadjoint dilation $S$ of dissipative operator $A$ is constructed using the knot $\Theta$ in [1] in the following manner.
The spaces ${H}_{-} = {L}_{2} \left( \left( -\infty, 0 \right], {E}_{-} \right)$, ${H}_{+} = {L}_{2} \left( \left[0, +\infty\right) \right)$ and $H = {H}_{-} \oplus \mathfrak{H} \oplus {H}_{+}$ are considered.
The vector $h = \left({h}_{-}, {h}_{0}, {h}_{+}\right) \in \mathfrak{D}\left(S\right)$ if and only if
Theorem. The dilation $S$ is minimal, i.e.
$$H = \overline{span\left\{ {R}_{\pm i} \left(S\right) h \mid h \in \mathfrak{H}, n \in \left\{0\right\} \cup \mathbb{N} \right\}}$$
if the spaces ${E}_{+} = \overline{\Psi \mathfrak{H}}$, ${E}_{-} = \overline{{\Phi}^{*}\mathfrak{H}}$ are separable.
The following expressions was used for the proof:
$$ {R}^{n}_{-i}\left(S\right) \begin{pmatrix} 0\\ {h}_{0}\\ 0 \end{pmatrix} = \begin{pmatrix} 0\\ {a}_{n}\\ {b}_{n} \end{pmatrix}, $$
where $n \in \mathbb{N}$, ${a}_{n} = {R}^{n} {h}_{0}$,
$${b}_{n} = {e}^{-t} \sum\limits_{k=1}^{n} \frac{{t}^{n-k}}{\left(n-k\right)! {i}^{n-k-1}} \Psi {R}^{k-1} {h}_{0}.$$

$$ {R}^{n}_{i} \left(S\right) \begin{pmatrix} 0 \\ {h}_{0} \\ 0 \end{pmatrix} = \begin{pmatrix} {c}_{n} \\ {d}_{n} \\ 0 \end{pmatrix}, $$
where ${d}_{n} = {{R}^{*}}^{n} {h}_{0}$,
$${c}_{n} = {e}^{t} \sum\limits_{k=1}^{n} \frac{{t}^{n-k}}{\left(n-k\right)! {\left(-i\right)}^{n-k-1}} {\Phi}^{*} {{R}^{*}}^{k-1} {h}_{0},$$
where ${h}_{0} \in \mathfrak{H}$.

Keywords: dilation, self-adjoint operator, unbounded dissipative operator, minimality, operatorknot.

UDC: 517.432

MSC: 47A48, 47A20



© Steklov Math. Inst. of RAS, 2024