Monoidal centres and groupoid-graded categories

TL;DR

This paper explores the structure of monoidal centres in bicategories related to categories and groupoids, introducing a higher-dimensional full centre concept and connecting it to group-graded categories used in topological invariants.

Contribution

It defines a higher-dimensional full monoidal centre for monoidales in monoidal bicategories and relates these structures to groupoid fibrations and group-graded categories.

Findings

Introduces the concept of a full monoidal centre of a monoidale in a monoidal bicategory.

Shows how groupoid fibrations provide examples of full monoidal centres.

Connects group-graded structures to monoidales in the monoidal centre, relevant for topological invariants.

Abstract

We denote the monoidal bicategory of two-sided modules (also called profunctors, bimodules and distributors) between categories by $\mathrm{Mod}$; the tensor product is cartesian product of categories. For a groupoid $\scr{G}$, we study the monoidal centre $\mathrm{ZPs}(\scr{G},\mathrm{Mod}^{\mathrm{op}})$ of the monoidal bicategory $\mathrm{Ps}(\scr{G},\mathrm{Mod}^{\mathrm{op}})$ of pseudofunctors and pseudonatural transformations; the tensor product is pointwise. Alexei Davydov defined the full centre of a monoid in a monoidal category. We define a higher dimensional version: the full monoidal centre of a monoidale (= pseudomonoid) in a monoidal bicategory $\scr{M}$, and it is a braided monoidale in the monoidal centre $\mathrm{Z}\scr{M}$ of $\scr{M}$. Each fibration $\pi : \scr{H} \to \scr{G}$ between groupoids provides an example of a full monoidal centre of a monoidale in $\mathrm{Ps}(\scr{G},\mathrm{Mod}^{\mathrm{op}})$. For a group $G$, we explain how the $G$-graded categorical structures, as considered by Turaev and Virelizier in order to construct topological invariants, fit into this monoidal bicategory context. We see that their structures are monoidales in the monoidal centre of the monoidal bicategory of $k$-linear categories on which $G$ acts.