Framed bicategories and monoidal fibrations

TL;DR

This paper introduces framed bicategories, a new categorical framework that better captures structures like bimodules and spans, and provides methods to construct them from monoidal fibrations, unifying various categorical concepts.

Contribution

It develops the theory of framed bicategories, offering new constructions from monoidal fibrations and extending the framework to include enriched and internal categories.

Findings

Framed bicategories effectively handle examples like bimodules and spans.

New constructions from monoidal fibrations produce framed bicategories.

Unified approach encompasses enriched and internal categories.

Abstract

In some bicategories, the 1-cells are `morphisms' between the 0-cells, such as functors between categories, but in others they are `objects' over the 0-cells, such as bimodules, spans, distributors, or parametrized spectra. Many bicategorical notions do not work well in these cases, because the `morphisms between 0-cells', such as ring homomorphisms, are missing. We can include them by using a pseudo double category, but usually these morphisms also induce base change functors acting on the 1-cells. We avoid complicated coherence problems by describing base change `nonalgebraically', using categorical fibrations. The resulting `framed bicategories' assemble into 2-categories, with attendant notions of equivalence, adjunction, and so on which are more appropriate for our examples than are the usual bicategorical ones. We then describe two ways to construct framed bicategories. One is an analogue of rings and bimodules which starts from one framed bicategory and builds another. The other starts from a `monoidal fibration', meaning a parametrized family of monoidal categories, and produces an analogue of the framed bicategory of spans. Combining the two, we obtain a construction which includes both enriched and internal categories as special cases.