Australian Category Seminar

Polynomial comodules and PRA functors

Richard Garnerยท12 March 2025

If V\mathcal{V} is a monoidal category whose tensor product preserves reflexive coequalisers in each variable, then there is a bicategory Comod(V)\mathbf{Comod}(\mathcal{V}) of comonoids and comodules in V\mathcal{V}. An example of this is where V=Poly\mathcal{V} = \mathbf{Poly}, the category of polynomial endofunctors of Set\mathbf{Set}, for which it turns out that Comod(Poly)\mathbf{Comod}(\mathbf{Poly}) is equivalent to PRA\mathbf{PRA}, the bicategory of parametric right adjoint functors between presheaf categories. I figured this out in 2020 in a very hands on way; this talk is about the slick way of doing this via the paper best known around here as [18].

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