The inner kernel theorem for a certain Segal algebra

TL;DR

This paper develops an inner kernel theorem for Segal algebras on locally compact Abelian groups, characterizing regularizing operators with kernels in the algebra, and applies this to operator composition and frequency analysis.

Contribution

It introduces the inner kernel theorem for Segal algebras, characterizing operators with kernels in the algebra without using Wilson bases, and explores their composition and approximation properties.

Findings

Characterization of regularizing operators with kernels in Segal algebra

Description of operator composition laws

Mathematically rigorous derivation of frequency integration to Dirac delta

Abstract

The Segal algebra $\mathbf{S}_{0}(G)$ is well defined for arbitrary locally compact Abelian Hausdorff (LCA) groups $G$. It is a Banach space that exhibits a kernel theorem similar to the well-known Schwartz kernel theorem. Specifically, we call this characterization of the continuous linear operators from $\mathbf{S}_{0}(G_{1})$ to $\mathbf{S}'_{0}(G_{2})$ by generalized functions in $\mathbf{S}'_{0}(G_{1} \times G_{2})$ the "outer kernel theorem". The main subject of this paper is to formulate what we call the "inner kernel theorem". This is the characterization of those linear operators that have kernels in $\mathbf{S}_{0}(G_{1} \times G_{2})$. Such operators are regularizing -- in the sense that they map $\mathbf{S}'_{0}(G_{1})$ into $\mathbf{S}_{0}(G_{2})$ in a $w^{*}$ to norm continuous manner. A detailed functional analytic treatment of these operators is given and applied to the case of general LCA groups. This is done without the use of Wilson bases, which have previously been employed for the case of elementary LCA groups. We apply our approach to describe natural laws of composition for operators that imitate those of linear mappings via matrix multiplications. Furthermore, we detail how these operators approximate general operators (in a weak form). As a concrete example, we derive the widespread statement of engineers and physicists that pure frequencies "integrate" to a Dirac delta distribution in a mathematically justifiable way.