Let $(\mathcal{C}, \otimes, e)$ be a monoidal category, and consider the category $[\mathcal{C}^\text{op}, \text{Cat}]$ of $\text{Cat}$-valued presheaves and natural transformations. I want to:

  1. Find an specialized definition of Day convolution for $[\mathcal{C}^\text{op}, \text{Cat}]$; and
  2. Find an explicit/concrete description of it.

Finding a specialized definition

I'm aware of the standard definition of Day convolution in terms of $\mathcal{V}$-enriched categories. I have also seen the coend formula when $\mathcal{V} = \text{Set}$. However, I do not know any enriched category theory and so cannot at the moment make use of the standard definition.

I also know that $\text{Cat}$-enriched categories are the same as (strict) $2$-categories, but I haven't found any definitions of Day convolution for $2$-categories (which hopefully would be easier to learn than enriched category theory).

In conclusion, I'm looking for a pre-existing specialized definition to $\mathcal{V} = \text{Cat}$ (and thus $\mathcal{C}$ is given the standard/trivial enrichment as $\text{Cat}$ is cocomplete) or a $2$-categorical definition.

Finding an explicit/concrete description

I've successfully applied the definition of Day convolution in the case of $\text{Set}$-valued presheaves. I expanded the coend formula to what it means in terms of universal properties. I also wrote it down as an explicit coequalizer.

In the end I had a description of the resulting set in terms of an equivalence relation generated by the two actions of the morphism on the profunctor I took the coend over.

The above is the level of concreteness I'm looking for.

What I'd like

For (1) I'd appreciate one of

  • a link if a definition exists; or
  • a quick working out of the general definition in the special case of $\text{Cat}$ if it's trivial for you; or
  • telling me I should just invest my time in learning enough enriched category theory to understand the standard definition.

For (2) if someone has a definition, an explicit description of the resulting category (objects and morphisms only is fine).


1 Answer 1


As a follow up, I spent some time learning enriched category theory, enough to take enriched coends. Thus, I could determine what Day convolution was as a $\text{Set}$-valued map from $[\mathcal{C}^\text{op}, \text{Cat}] \times [\mathcal{C}^\text{op}, \text{Cat}] \to [\mathcal{C}^\text{op}, \text{Cat}]$, which was enough for now.


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