Coherence for Categorified Operadic Theories

TL;DR

This paper introduces a general framework for categorifying algebraic theories described by operads, allowing equations to hold up to coherent isomorphism, and extends classical strictification results to these generalized theories.

Contribution

It defines a notion of weakening for operadic theories, generalizes strictification results, and proves the independence of categorification from presentation choices.

Findings

Every monoidal category is monoidally equivalent to a strict one.

The strictification functor is left adjoint to the forgetful functor.

Categorification is independent of the presentation of the operad.

Abstract

Given an algebraic theory which can be described by a (possibly symmetric) operad $P$, we propose a definition of the \emph{weakening} (or \emph{categorification}) of the theory, in which equations that hold strictly for $P$-algebras hold only up to coherent isomorphism. This generalizes the theories of monoidal categories and symmetric monoidal categories, and several related notions defined in the literature. Using this definition, we generalize the result that every monoidal category is monoidally equivalent to a strict monoidal category, and show that the "strictification" functor has an interesting universal property, being left adjoint to the forgetful functor from the category of strict $P$-categories to the category of weak $P$-categories. We further show that the categorification obtained is independent of our choice of presentation for $P$, and extend some of our results to many-sorted theories, using multicategories.