Frames, semi-frames, and Hilbert scales

TL;DR

This paper explores semi-frames in Hilbert spaces, introducing Hilbert scales for upper semi-frames, analyzing duality, and extending concepts to fusion and Banach semi-frames, with applications in signal processing.

Contribution

It introduces Hilbert scales associated with semi-frames, providing new characterizations of function spaces and extending semi-frame theory to fusion and Banach contexts.

Findings

Hilbert scales for upper semi-frames are constructed.

Duality between lower and upper semi-frames is analyzed.

Extensions to fusion and Banach semi-frames are presented.

Abstract

Given a total sequence in a Hilbert space, we speak of an upper (resp. lower) semi-frame if only the upper (resp. lower) frame bound is valid. Equivalently, for an upper semi-frame, the frame operator is bounded, but has an unbounded inverse, whereas a lower semi-frame has an unbounded frame operator, with bounded inverse. For upper semi-frames, in the discrete and the continuous case, we build two natural Hilbert scales which may yield a novel characterization of certain function spaces of interest in signal processing. We present some examples and, in addition, some results concerning the duality between lower and upper semi-frames, as well as some generalizations, including fusion semi-frames and Banach semi-frames.