Dynamical Frames and Hyperinvariant Subspaces

TL;DR

This paper characterizes the uniqueness of dynamical frame representations for semigroups using co-hyperinvariant subspaces, extending previous results and addressing the case of multiple commuting operators.

Contribution

It provides a general characterization of central frame representations for any semigroup, unifying and extending prior results on shift-invariant subspaces.

Findings

All frame generators of a semigroup from commuting operators are equivalent.

The characterization applies to semigroups generated by any k-tuple of commuting operators.

The results connect to the structure of co-hyperinvariant subspaces in Hardy spaces.

Abstract

The theory of dynamical frames evolved from practical problems in dynamical sampling where the initial state of a vector needs to be recovered from the space-time samples of evolutions of the vector. This leads to the investigation of structured frames obtained from the orbits of evolution operators. One of the basic problems in dynamical frame theory is to determine the semigroup representations, which we will call central frame representations, whose frame generators are unique (up to equivalence). Recently, Christensen, Hasannasab, and Philipp proved that all frame representations of the semigroup $\Bbb{Z}_{+}$ have this property. Their proof of this result relies on the characterization of the structure of shift-invariant subspaces in $H^2(\mathbb{D})$ due to Beurling. In this paper we settle the general uniqueness problem by presenting a characterization of central frame representations for any semigroup in terms of the co-hyperinvariant subspaces of the left regular representation of the semigroup. This result is not only consistent with the known result of Han-Larson in 2000 for group representation frames, but also proves that all the frame generators of a semigroup generated by any $k$-tuple $(A_1, ... A_k)$ of commuting bounded linear operators on a separable Hilbert space $H$ are equivalent, a case where the structure of shift-invariant subspaces, or submodules, of the Hardy Space on polydisks $H^{2}(\Bbb{D}^k)$ is still not completely characterized.