Let be an abelian category with enough projectives. In the previous post, we described the definition of the *derived -category* of . As a simplicial category, this consisted of bounded-below complexes of projectives, and the space of morphisms between two complexes was obtained by taking the *chain complex* of maps between and turning that into a space (by truncation and the Dold-Kan correspondence).

Last time, we proved most of the following result:

Theorem 5is a stable -category whose suspension functor is given by shifting by . has a -structure whose heart is , and the homotopy category of is the usual derived category.

Note for instance that this means that sits as a full subcategory inside : that is, there is a full subcategory (the “heart”) of (spanned by those complexes homologically concentrated in degree zero).

This heart has the property that the mapping spaces in are discrete, and the functor

restricts to an equivalence ; one can prove this by examining the chain complex of maps between two complexes homologically concentrated in degree zero. The inverse to this equivalence runs , and it sends an element of to a projective resolution. This is functorial in the -categorical sense.

Most of the above theorem is exactly the same as the description of the ordinary derived category of (i.e., the homotopy category of ), The goal of this post is to describe what’s special to the -categorical setting: that there is a universal property. I will start with the universal property for the subcategory .

Theorem 6is the -category obtained from (the projective objects) by freely adding geometric realizations.

The purpose of this post is to sketch a proof of the above theorem, and to explain what it means.

**1. Freely adding colimits**

Let be an ordinary category, and a cocomplete category. Let be the category of presheaves on . Then it is a classical fact that we have an equivalence of categories

that is, to give a functor is equivalent to giving a *colimit-preserving* (equivalently, left adjoint) functor . In other words, is the *free cocompletion* of , obtained by adding all colimits freely. The Yoneda embedding

is the universal imbedding of into a cocomplete category.

There is an analogous result in the -categorical setting. Let be an -category. We can define the analogous -category

where is the -category of spaces. Then, if is any cocomplete -category, there is an equivalence of -categories,

between functors and colimit-preserving functors .

We will need a variant of the above construction, when we freely add only some colimits rather than all of them. For simplicity, we state the next result for geometric realizations.

Proposition 7Let be an -category. Then there exists an -category admitting geometric realizations, together with an inclusion with the property that for any -category admitting geometric realizations, we have an equivalence

between functors and functors that preserve geometric realizations.

The strategy is to start by taking to be the smallest sub-category of which contains the image of the Yoneda imbedding and which is closed under taking geometric realizations. Then, clearly contains ; to complete the proof, one has to show that a functor

is a left Kan extension of its restriction to if and only if it preserves geometric realizations. (“Kan extension” seems to be one of the magic words one has to throw around in this business, to make the proofs work efficiently.) In fact, one has an equivalence between functors and left Kan extensions , provided that left Kan extensions exist.

So let’s prove this precisely:

- A functor admits a left Kan extension (if admits geometric realizations).
- A functor is a left Kan extension of its restriction to precisely if it preserves geometric realizations.

How can we prove this? The strategy, for 1, to show that the left Kan extension exists, is to note that it would exist perfectly well if we knew was cocomplete. But we can find a colimit-preserving imbedding

into a cocomplete -category (for instance, the opposite to the Yoneda imbedding), and any functor admits a left Kan extension to . But, since is generated under geometric realizations from , one can then check that is contained (up to isomorphism) in itself. This means that the left Kan extension exists, and in fact is a special case of a left Kan extension

In particular, we’ve established the first step. Observe that the Kan extension constructed in this way preserves geometric realizations, because the functor was constructed so as to preserve colimits. (This is part of the yoga of the Yoneda lemma: Yoneda extensions are left Kan extensions, and preserve colimits.)

Now we need to establish the second step. That is, if is any functor preserving geometric realizations, then it is a left Kan extension of its restriction to . We can do this by observing that a left Kan extension of its restriction *exists*, by the previous step, and said left Kan extension preserves geometric realizations.

I’ve sort of sketched this business; it’s in 5.3 of “Higher Topos Theory.” I’m not sure how much going into these details would be relevant to a blog setting.

Definition 8Given an -category , we let be the -category obtained by freely adding geometric realizations, as above.

**2. Proof of the universal property**

With the general formalism done, we can turn to the main goal, which was to say that was obtained by adding geometric realizations to the -category (even ordinary category) of projectives .

How might we prove this? Roughly, the intuition is that consists of nonnegatively graded chain complexes with elements in and these are obtained from simplicial objects by Dold-Kan.

To make it precise, let’s first construct a functor. One can show that admits geometric realizations. Let’s accept this—the idea is that it certainly admits finite colimits (being stable) but taking higher and higher stages of a geometric realization only affect the “tail end” (the connectivity increases), so turns out to have geometric realizations. (This is a “left completeness” condition.)

Anyway, there is certainly an imbedding

and since the latter -category admits geometric realizations, we get a geometric-realization-preserving functor

which we have to prove is an equivalence.

To prove that is an equivalence, we have to show that is fully faithful and essentially surjective. Let’s start with full faithfulness. Certainly is fully faithful— is a full subcategory of .

Proposition 9is fully faithful.

*Proof:* Let’s start by settling a special case of full faithfulness.

Given an object , we can ask for the collection of such that

is an equivalence. As we’ve seen, this contains all . The idea is that is a “projective” object, so this collection of will be stable under geometric realizations, and thus contain all of . If we can prove it, then we will be done with full faithfulness: in fact, the collection of all for which

is an equivalence (for all ) is closed under geometric realizations.

Why is this collection of objects stable? We have to show that the functors into spaces and commute with geometric realizations. In (which is contained in a presheaf category), that’s a special case of the Yoneda principle: homming out of a representable functor commutes with colimits in the presheaf category (e.g., geometric realizations), because homming out of a representable functor is equivalently evaluation. In the case of , it is a little more subtle.

The point is that is concentrated in degree zero: it is the complex with some (projective) in degree zero and zero elsewhere. Given a complex , we have that the internal (chain complex) homs are given by

Consider now a simplicial object in . For simplicity, assume that it is a simplicial object in the 1-category of chain complexes (i.e., on the nose, not up to coherent homotopy); this can always be arranged suitably, possibly by changing the category up to equivalence. So is really a simplicial chain complex, or a double chain complex . The geometric realization, spaces or chain complexes, corresponds to taking the total complex. But this is all preserved under taking .

The above proof was somewhat handwavy. One key point near the end is that the functor from nonnegatively graded chain complexes (i.e., simplicial abelian groups) to spaces preserves geometric realizations. This is an illustration of the fact that simplicial abelian groups are *monadic* over spaces.

Anyway, the point of the above argument is that we get a fully faithful functor

and the last thing to check is that it is essentially surjective. But this is the Dold-Kan correspondence. Given an object in , it is a chain complex of projectives. Dold-Kan allows us to say that it corresponds to a simplicial object in , and in fact the object is a geometric realization of that simplicial object. So is generated under geometric realizations by , which means that

is essentially surjective, and hence an equivalence.

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