Transformations of Nevanlinna operator-functions and their fixed points

Abstract

We give a new characterization of the class ${\bf N}^0_{\mathfrak M}[-1,1]$ of the operator-valued in the Hilbert space ${\mathfrak M}$ Nevanlinna functions that admit representations as compressed resolvents ($m$-functions) of selfadjoint contractions. We consider the automorphism ${\bf Γ}:$ $M(λ){\mapsto}M_{\bf Γ}(λ):=\left((λ^2-1)M(λ)\right)^{-1}$ of the class ${\bf N}^0_{\mathfrak M}[-1,1]$ and construct a realization of $M_{\bf Γ}(λ)$ as a compressed resolvent. The unique fixed point of ${\bfΓ}$ is the $m$-function of the block-operator Jacobi matrix related to the Chebyshev polynomials of the first kind. We study a transformation ${\bf\widehat Γ}:$ ${\mathcal M}(λ)\mapsto {\mathcal M}_{\bf\widehat Γ}(λ) :=-({\mathcal M}(λ)+λI_{\mathfrak M})^{-1}$ that maps the set of all Nevanlinna operator-valued functions into its subset. The unique fixed point $\mathcal M_0$ of ${\bf\widehatΓ}$ admits a realization as the compressed resolvent of the "free" discrete Schrödinger operator ${\bf\widehat J}_0$ in the Hilbert space ${\bf H}_0=\ell^2(\mathbb N_0)\bigotimes{\mathfrak M}$. We prove that ${\mathcal M}_0$ is the uniform limit on compact sets of the open upper/lower half-plane in the operator norm topology of the iterations $\{{\mathcal M}_{n+1}(λ)=-({\mathcal M}_n(λ)+λI_\mathfrak M)^{-1}\}$ of ${\bf\widehatΓ}$. We show that the pair $\{{\bf H}_0,{\bf \widehat J}_0\}$ is the inductive limit of the sequence of realizations $\{\widehat{\mathfrak H}_n,\widehat A_n\}$ of $\{{\mathcal M}_n\}$. In the scalar case $({\mathfrak M}={\mathbb C})$, applying the algorithm of I.S.~Kac, a realization of iterates $\{{\mathcal M}_n\}$ as $m$-functions of canonical (Hamiltonian) systems is constructed.