Frames of subspaces in Hilbert spaces with $W$-metrics

TL;DR

This paper studies the behavior of frames of subspaces in Hilbert spaces equipped with a $W$-metric, exploring their structure, decomposition, and dynamics, especially when the operator $W$ is unbounded.

Contribution

It introduces a decomposition of $W$-metric Hilbert spaces into Krein spaces and analyzes the dynamics of frames of subspaces in these contexts, extending previous work.

Findings

Decomposition of $ ext{W}$-space into Krein spaces.

Behavior of frames in $W$-spaces with unbounded $W$.

Structural insights into $W$-metric Hilbert spaces.

Abstract

If $\left(\h,\langle\cdot,\cdot\rangle\right)$ is a Hilbert space and on it we consider the sesquilinear form $\langle\,W\cdot,\cdot\rangle$ so-called $W$-metric, where $W^{*}=W\in\BH$, and $\ker\,W=\{0\}$, then the space $\left(\h,\langle\,W\cdot,\cdot\rangle\right)$ is called Hilbert space with $W$-metric or simply $W$-space. In this paper we investigate the dynamic of frames of subspace on these spaces, where the sense of dynamics refers to the behavior of frames of subspace in $\h_{W}$ (the completion of $\left(\h,\langle\,W\cdot,\cdot\rangle\right)$) comparing with $\h$ and vice versa. This work is based on the study made in \cite{KEFER,GMMM} on frames in Krein spaces. In a similar way, Casazza and Kutyniok obtained some results in the context of Hilbert spaces, see \cite{CG}. We take tools of theory of $C^{*}$-algebra, and properties of $\BH$, to show that every Hilbert space with $W$-metric $\h_{W}$ with $0\in\sigma(W)$ has a decomposition $$\h_{W}=\bigoplus_{n\in\N\cup\{\infty\}}\h_{\psi_{n}}^{W},$$ where $\h_{\psi_{n}}^{W}\simeq \Ele(\sigma(W),x\,d\mu_{n}(x))$ are Krein spaces, for every $n\in\N\cup\{\infty\}$. Moreover, we investigate the dynamics of frames of subspace when the self-adjoint operator $W$ is unbounded.