nLab totally distributive category

Definition

Recall that a locally small category 𝒦\mathcal{K} is total if its Yoneda embedding Y:π’¦βŸΆ[𝒦 op,Set]Y\colon\mathcal{K}\longrightarrow [\mathcal{K}^{op},Set] has a left adjoint XX.

Definition

A total category 𝒦\mathcal{K} is totally distributive if X:[𝒦 op,Set]βŸΆπ’¦X \colon [\mathcal{K}^{op},Set] \longrightarrow \mathcal{K} has a further left adjoint WW.

(W⊣X⊣Y):𝒦β†ͺY←Xβ†ͺWPSh(𝒦). (W \dashv X \dashv Y) \;\colon\; \mathcal{K} \stackrel{\overset{W}{\hookrightarrow}}{\stackrel{\overset{X}{\leftarrow}}{\underset{Y}{\hookrightarrow}}} PSh(\mathcal{K}) \,.

Properties

If 𝒦\mathcal{K} is totally distributive, then since YY is fully faithful, then, by the properties of adjoint triples, so is WW. Thus, 𝒦\mathcal{K} is a coreflective subcategory of [𝒦 op,Set][\mathcal{K}^{op},Set], which is cototal (or more precisely, pro-cototal?, since it is not locally small) β€” hence 𝒦\mathcal{K} is also cototal.

Moreover this means that the induced adjoint pair of (co-)monads

WX⊣YX:PSh(𝒦)↔PSh(𝒦) W X \dashv Y X \;\colon\; PSh(\mathcal{K}) \leftrightarrow PSh(\mathcal{K})

is an adjoint modality.

Every totally distributive category is an example of a completely distributive category.

Examples

Waves

For any total category, it is possible to define the functor W:𝒦→[𝒦 op,Set]W:\mathcal{K} \to [\mathcal{K}^{op},Set] which β€œwants to be” the left adjoint of XX. The elements of W(A)(K)W(A)(K) are called waves from KK to AA (just because they are usually denoted by wavy arrows), and as in the case of continuous categories they form an idempotent comonad on 𝒦\mathcal{K} in the bicategory of profunctors. A total category is totally distributive if and only if W⊣XW \dashv X.

References

Last revised on January 7, 2025 at 16:07:21. See the history of this page for a list of all contributions to it.