Let $C_1$ and $C_2$ be monoidal categories (not necessarily symmetric or strict) and let $\Psi : C_1 \rightarrow C_2$ be a strong monoidal functor. Is it possibly to construct a strict monoidal functor $\Psi'$ which is naturally isomorphic to $\Psi$? If not, is this possible if we assume that the $C_i$ are strict?

$\begingroup$ Not an answer, but in the general context of 2monads, a (strict) algebra $A$ with the property that every pseudo morphism with domain $A$ (and strict codomain) is isomorphic to a strict one is called semiflexible. See BlackwellKellyPower, 2dimensional monad theory, Theorem 4.7. In general, not every algebra is semiflexible, and that is probably also the case here. $\endgroup$– Mike ShulmanJun 29, 2014 at 6:29
2 Answers
Take $C_2$ to be a nonstrict monoidal category, $C_1$ to be its strictification and $\Psi$ to be the equivalence. Since $C_2$ is not strict there's no strict monoidal equivalence between $C_1$ and $C_2$, so in particular $\Psi$ is not naturally isomorphic to a strict functor.

$\begingroup$ Great, I should have noticed that. I'm going to hold off on accepting it to see if someone answers the second part of my question (namely, what if we assume that both of the $C_i$ are strict). $\endgroup$– JillJun 27, 2014 at 18:49
I think the answer to the second part (both $C_i$ strict) is negative.
Fix a finite group $G$ with a nontrivial 2cocycle $\gamma \in H^2(G, \mathbb{C}^{\times})$. Let $C$ be the strict $\mathbb{C}$linear monoidal category with simple objects $\{g: g \in G\}$ and tensor product $g \otimes h = gh$. Then the 2cocycle $\gamma$ can be used to define a monoidal functor $(1_C, \gamma): C \to C$. This functor is not naturally isomorphic to a strict functor (the only possibility is it is isomorphic to the (strict) identity functor, but this cannot be the case since $\gamma$ is nontrivial).

$\begingroup$ Could you please say a little bit more how the functor $(1_C,\gamma):C\to C$ is defined? $\endgroup$– jijijojoMay 7, 2020 at 21:35

3$\begingroup$ The functor $(1_C, \gamma)$ is the identity on objects and morphisms, and the monoidal structure maps $g \otimes h \to g \otimes h$ are given by $\gamma(g,h)$ times the identity of the object $gh$ (the cocycle condition ensures that the axioms for a monoidal functor are satisfied, and two cocycles which differ by a coboundary give monoidally naturally isomorphic functors) $\endgroup$ May 8, 2020 at 22:34