Spectrum of normal operators that generate certain scalable iterative systems
Pu-Ting Yu
TL;DR
This work characterizes spectral restrictions on normal operators $A$ that generate finite-atom iterative systems which can be rescaled to form frames for an infinite-dimensional Hilbert space $H$. Under a syndetic-subsequence growth condition on the rescaling coefficients, the continuous spectrum $ ext{σ}_c(A)$ is shown to be confined to finitely many circular arcs centered at the origin, with at most $N\le |S|-1$ radii, and forces $A$ to be diagonal when $S$ is a singleton. These results yield partial progress on Aldroubi's conjecture by proving non-frame-ness of the normalized iterative system whenever $ ext{σ}_c(A)$ contains more than $|S|-1$ distinct moduli or when $S=\{x\}$ and $A$ is non-diagonal. The approach blends the spectral theorem for normal operators, frame theory, and measure-theoretic spectral analysis to derive concrete spectral obstructions to rescaled-iterative frames. The findings advance understanding of time-to-space sampling tradeoffs in infinite dimensions and provide explicit spectral conditions that preclude forming frames from normalized iterative systems.
Abstract
Let $A\colon H\rightarrow H$ be a normal operator on an infinite-dimensional separable Hilbert space $H$ and let $S\subseteq H$ be a finite subset such that $\{A^nx\}_{n\geq 0,\,x\in S}$ can be rescaled to form a frame for $H$. That is, there exist some subsets $J_x\subseteq \mathbb{N}\cup\{0\}$ and some set of nonzero scalars $(c_{n,x})_{n\in J_x,\,x\in S}$ such that $\{c_{n,x}A^nx\}_{n\in J_x,\,x\in S}$ forms a frame for $H.$ Assume that there exist some $η\in\mathbb{N}$ and $δ>0$ such that for each infinite $J_x$ there is an increasing syndetic subsequence $(n^x_{k})_{k\in \mathbb{N}}\subseteq J_x$ satisfying $|c_{n^x_{k},x}|\|A^{i^x_{k}}x\|\geq δ$ for some non-negative integers $i^x_{k}$ with $|i^x_{k}- n^x_{k}|\leq η$ for all $k\in \mathbb{N}$. We prove that there exist finitely many numbers $(r_i)_{i=1}^N$ such that the continuous spectrum of $A$ is concentrated on arcs of a circle centered at origin with radius $r_i$. In particular, $A$ must be a diagonal operator if $S$ is a singleton. As an application, we establish the conjecture proposed by Aldroubi et al.\ asserting that the iterative system $\{\frac{A^nx}{\|A^nx\|}\}_{n\geq 0,\,x\in S}$ is never a frame for $H$, provided one of the following two conditions holds: (i) The continuous spectrum of $A$ contains more than $|S|-1$ points with distinct moduli; (ii) $S$ is a singleton and $A$ is not a diagonal operator