# Sectional algebras of semigroupoid bundles

## TL;DR

This paper develops a framework using semigroupoids to describe algebraic bundles and their sectional algebras, generalizing many constructions and establishing isomorphisms with crossed products, including in non-Hausdorff settings.

## Contribution

It introduces a unified approach to sectional algebras of bundles via semigroupoids, generalizes smash products to groupoid graded algebras, and extends results to non-Hausdorff cases.

## Key findings

- Semigroupoid bundles correspond to various algebraic constructions.
- Generalization of smash products to groupoid graded algebras.
- Isomorphism between Steinberg algebra of germs and crossed products.

## Abstract

In this article we use semigroupoids to describe a notion of algebraic bundles, mostly motivated by Fell ($C^*$-algebraic) bundles, and the sectional algebras associated to them. As the main motivational example, Steinberg algebras may be regarded as the sectional algebras of trivial (direct product) bundles. Several theorems which relate geometric and algebraic constructions - via the construction of a sectional algebra - are widely generalized: Direct products bundles by semigroupoids correspond to tensor products of algebras; Semidirect products of bundles correspond to "na\"ive" crossed products of algebras; Skew products of graded bundles correspond to smash products of graded algebras; Quotient bundles correspond to quotient algebras. Moreover, most of the results hold in the non-Hausdorff setting. In the course of this work, we generalize the definition of smash products to groupoid graded algebras. As an application, we prove that whenever $\theta$ is a $\land$-preaction of a discrete inverse semigroupoid $S$ on an ample (possibly non-Hausdorff) groupoid $\mathcal{G}$, the Steinberg algebra of the associated groupoid of germs is naturally isomorphic to a crossed product of the Steinberg algebra of $\mathcal{G}$ by $S$. This is a far-reaching generalization of analogous results which had been proven in particular cases.