Not every pseudoalgebra is equivalent to a strict one

TL;DR

The paper demonstrates that not all pseudoalgebras of certain 2-monads can be strictified, using higher category theory examples like semi-strict 3-categories and braided monoidal categories.

Contribution

It provides a counterexample showing that having rank is not sufficient for strictification of pseudoalgebras in 2-category theory.

Findings

Not every Gray-category is equivalent to a strict 3-category

Counterexample from higher category theory involving semi-strict 3-categories

Braided monoidal categories yield pseudoalgebras that are not strictifiable

Abstract

We describe a finitary 2-monad on a locally finitely presentable 2-category for which not every pseudoalgebra is equivalent to a strict one. This shows that having rank is not a sufficient condition on a 2-monad for every pseudoalgebra to be strictifiable. Our counterexample comes from higher category theory: the strict algebras are strict 3-categories, and the pseudoalgebras are a type of semi-strict 3-category lying in between Gray-categories and tricategories. Thus, the result follows from the fact that not every Gray-category is equivalent to a strict 3-category, connecting 2-categorical and higher-categorical coherence theory. In particular, any nontrivially braided monoidal category gives an example of a pseudoalgebra that is not equivalent to a strict one.