Positive positive-definite functions and measures on locally compact abelian groups

TL;DR

This paper explores the properties and structures of functions and measures on locally compact abelian groups that are both positive and positive-definite, addressing a previously under-studied area with implications for harmonic analysis and number theory.

Contribution

It systematically studies positive and positive-definite functions and measures on locally compact abelian groups, filling a gap in the literature and analyzing their operations and structures.

Findings

Identified properties of these functions and measures under certain integrability conditions.

Explored operations and constructions involving these functions and measures.

Highlighted open questions about the structure of the cone of such functions, even on simple groups like .

Abstract

The author was recently able to provide a cohomological interpretation of Tate's Riemann-Roch formula for number fields using some new harmonic analysis objects, ghost-spaces. When trying to investigate these objects in general, we realized the importance of functions and measures on locally compact abelian groups that are both positive and positive-definite at the same time. It looks like this class of functions and measures was not systematically studied before. The goal of this paper is to partially fill in this gap. We answer some of the natural questions involving these functions and measures, especially those that satisfy some extra integrability conditions. We also study some operations and constructions involving these functions and measures. There are several very interesting open questions, that we are only able to point out at this moment. In particular, the structure of the cone of such functions is not clear even when the group is just $\R.$