category theory

Yoneda lemma

# Contents

## Definition

###### Definition

A locally small category $C$ is total if its Yoneda embedding $Y \;\colon \;C\longrightarrow [C^{op},Set]$ has a left adjoint $L$.

If the opposite category $C^{op}$ is total, $C$ is called cototal.

###### Remark

The definition above requires some set-theoretic assumption to ensure that the functor category $[C^{op},Set]$ exists, but it can be rephrased to say that the colimit of $Id_C:C\to C$ weighted by $W$ exists, for any $W:C^{op}\to Set$. (This still involves quantification over large objects, however, so some foundational care is needed.) This version has an evident generalization to enriched categories.

###### Remark

Since the Yoneda embedding is a full and faithful functor, a total category $C$ induces an idempotent monad $Y \circ L$ on its category of presheaves, hence a modality. One says that $C$ is a totally distributive category if this modality is itself the right adjoint of an adjoint modality.

###### Remark

The $(L \dashv Y)$-adjunction of a total category is closely related to the $(\mathcal{O} \dashv Spec)$-adjunction discussed at Isbell duality and at function algebras on ∞-stacks. In that context the $L Y$-modality deserves to be called the affine modality.

## Properties

• Total categories satisfy a very satisfactory adjoint functor theorem: any colimit-preserving functor from a total category to a locally small category has a right adjoint.

• Although the definition refers explicitly only to colimits, every total category is also complete, i.e. has all small limits. It also has some large limits. In fact, it has “all possible” large limits that a locally small category can have: if $F\colon D\to C$ is a functor such that $lim_d Hom_C(X,F d)$ is a small set for all $X\in C$, then $F$ has a limit.

## Examples

Any cocomplete and epi-cocomplete category with a generator is total. (And more generally, any cocomplete and $E$-complete category with an $E$-generator is total, for a suitable class $E$.) See (Day), theorem 1, for a proof. This includes:

Also, totality lifts along solid functors; that is, if the codomain of a solid functor is total, then so is its domain. See (Tholen) for a proof. This implies that the following types of categories are total:

Thus, “most naturally-occurring” cocomplete categories are in fact total. However, cototality is more rare. But cototal categories do occur:

## References

• Ross Street, Bob Walters, Yoneda structures on 2-category, (contains the original definition of total categories)

• Max Kelly, A survey of totality for enriched and ordinary categories, Cahiers de Topologie et Géométrie Différentielle Catégoriques, 27 no. 2 (1986), p. 109-132, numdam

• Brian Day, Further criteria for totality, Cahiers de Topologie et Géométrie Différentielle Catégoriques, 28 no. 1 (1987), p. 77-78, numdam
• R.J Wood, Some remarks on total categories, J. Algebra 75_:2, 1982, 538–545 doi

Revised on March 6, 2014 08:56:36 by Zoran Škoda (161.53.130.104)