# Quantales and Fell bundles

## TL;DR

This paper explores the relationship between Fell bundles on étale groupoids and quantale theory, establishing a framework that connects algebraic, topological, and groupoid structures through C*-algebras and pseudogroups.

## Contribution

It introduces a map of involutive quantales linking the convolution algebra of Fell bundles to the topology of the groupoid, and studies properties related to the structure of the bundle and the groupoid.

## Key findings

- The map $p: ext{Max} ext{A} o ext{Ω}(G)$ mimics open maps of topological spaces.
- Under certain conditions, the algebra of sections coincides with the reduced C*-algebra.
- Max A is shown to be stably Gelfand, enabling the construction of associated étale groupoids.

## Abstract

We study Fell bundles on groupoids from the viewpoint of quantale theory. Given any saturated upper semicontinuous Fell bundle $\pi:E\to G$ on an \'etale groupoid $G$ with $G_0$ locally compact Hausdorff, equipped with a suitable completion C*-algebra $A$ of its convolution algebra, we obtain a map of involutive quantales $p:\mathrm{Max}\ A\to\Omega(G)$, where $\mathrm{Max}\ A$ consists of the closed linear subspaces of $A$ and $\Omega(G)$ is the topology of $G$. We study various properties of $p$ which mimick, to various degrees, those of open maps of topological spaces. These are closely related to properties of $G$, $\pi$, and $A$, such as $G$ being Hausdorff, principal, or topological principal, or $\pi$ being a line bundle. Under suitable conditions, which include $G$ being Hausdorff, but without requiring saturation of the Fell bundle, $A$ is an algebra of sections of the bundle if and only if it is the reduced C*-algebra $C_r^*(G,E)$. We also prove that $\mathrm{Max}\ A$ is stably Gelfand. This implies the existence of a pseudogroup $\mathcal{I}_B$ and of an \'etale groupoid $\mathfrak B$ associated canonically to any sub-C*-algebra $B\subset A$. We study a correspondence between Fell bundles and sub-C*-algebras based on these constructions, and compare it to the construction of Weyl groupoids from Cartan subalgebras.