Last time, we explained the idea of a **cofibration** in terms of a useful homotopy extension property. We showed that, for Hausdorff spaces, cofibrations turn out always to be closed immersions. Moreover, we showed that a pair with closed is a cofibration precisely when it satisfies a technical condition of being a neighborhood deformation retract. However, we have yet to give useful examples. The main result in this post is that a relative CW complex (for instance, a CW pair) leads to a cofibration.

**1. The mapping cylinder **

It turns out that, up to homotopy equivalence, every map is a cofibration. The method of showing this is to use the mapping cylinder. So let be a map. Recall that the **mapping cylinder** is the quotient space where is identified with . We have an inclusion map sending and a projection map sending .

The projection map is a homotopy equivalence. In fact, deformation retracts onto because deformation retracts onto . The homotopy inverse of the inclusion can be taken to be this projection. So we can factor via

where the second map is a homotopy equivalence.

Proposition 1is a cofibration.

*Proof:* Indeed, we can consider the function sending (and ). Then . Moreover, it is easy to see that deformation retracts onto . We can actually extend the deformation retraction to all of by making it go slower on as gets larger, and eventually stop moving. For instance, we could choose

It is thus easy to see that is an NDR pair, so the inclusion of in the mapping cylinder is a cofibration.

In particular, every map is equivalent in the homotopy category to a cofibration.

** General facts **

One of the key results one uses over and over again in homotopy theory is that a CW complex has the homotopy extension property with respect to any subcomplex. The reason is that it allows you to construct homotopies piece by piece.

More generally, we shall see this with a bit of categorical nonsense:

Proposition 2Consider a cocartesian diagram of topological spaces

Then if is a cofibration, so is .

Let us recall what the idea of a “cocartesian” diagram is. This means that is the push-out of with respect to . In other words, is the disjoint sum with the images of in both identified. To hom out of is the same thing as homming out such that the pull-backs to are compatible.

*Proof:* The reason is purely formal. Suppose has the homotopy lifting property and we have maps , which agree on . The first gives a map ; the second gives a map . These two agree on .

The homotopy lifting property says that we can extend and to . We thus have two maps which agree on . Now is the push-out of with respect to because push-outs (and more generally, quotients) commute with products with a locally compact Hausdorff space. (This is just one of those random facts in general topology that keep cropping up again and again.)

So since we have hommed out of into , we get a map . This extends . We need to just check that extends . But this is evident because the restriction of to is determined by the restrictions to , and these were done appropriately.

As a result, we can show that attaching a space by a cofibration leads to a cofibration. Since CW complexes are obtained by attaching cells, this is very nice.

Suppose is a topological space. Let be a subspace; suppose we have a map . Then we can consider the **attached** space

In particular, we tack onto and identify the points in with their images. The standard example of doing this is when . Then is said to be obtained from by attaching -cells. This process is how one can construct a so-called relative CW complex.

Proposition 3Suppose is an NDR pair (i.e. is a cofibration) and . Then is a cofibration.

*Proof:* Again, this can be done via categorical nonsense. In fact, the process of attaching spaces is a special case of the push-out. We can draw a cocartesian diagram

Indeed, this diagram is cocartesian precisely because is precisely what you get when you combine and and glue together the things coming from . And now it is clear that since is a cofibration, is as well.

** CW pairs **

Now, we have to show more generally that a relative CW-complex is an NDR pair. This will be of crucial importance as we continue discussing homotopy theory, because oftentimes we will construct homotopies piece-by-piece over various skeleta, and we will need to know that they can be extended.

So let’s recall what a **relative CW complex** is. This is a pair with a filtration

such that, for each , is obtained from from attaching -cells via maps , and such that is the inductive limit of the as a topological space. The last structure means in particular that has the weak topology on the : a subset of is open if and only if the intersections with each are open in . For instance, the inclusion of a subcomplex of a CW complex satisfies this.

Proposition 4Let be a relative CW complex. Then the inclusion

is a cofibration.

This is the main result of the post, and it will require a bit of preparation. First, let’s recall that in algebraic geometry, after every property (e.g. finite type, quasi-compactness, flatness, etc.) one has a long list of properties. These properties become a standard routine: property X is stable under base-change, preserved under composition, etc., etc. We have not done that for cofibrations. However, there is one such result which we should note.

Lemma 5Let be cofibrations. Then is a cofibration.

*Proof:* This is another purely formal statement. Suppose we have and which are compatible (i.e. agree on ). Then we have to extend the homotopy to . To do this, we start by extending it to (in such a way as to coincide with on ), then extend this to . So we use the homotopy extension property for each piece twice.

Now we are going to use the fact that relative CW complexes are obtained by a repeated attaching procedure. So we show:

Lemma 6Let be an indexing set. Then the map

is a cofibration.

*Proof:* It is easy to check, by the usual routine (and here I shall not spell out the details) that a disjoint union of cofibrations is a cofibration; this comes from the fact that the disjoint union is a coproduct in the category of topological spaces.

Anyway, this means that it’s enough to show that is a cofibration. For that, we note that the function on is such that deformation retracts onto . Moreover, we can arrange things so that the map extends to all of (but it is no longer a deformation retraction). In this way, we see that is an NDR pair.

Alternatively, we could show that is a deformation retract of . We can do this by “projecting” from a high point about the cylinder . I should draw a picture, but I am being lazy.

Anyway, suppose we have a relative CW complex . Then the previous two propositions together state that for each , the map is a cofibration. More generally, if , then is a cofibration. This is because it is a composite of attaching maps of disks along their boundaries; we know that composites preserve cofibrations; we know that the inclusion of the sphere into a disk is a cofibration; and we know that attaching leads to cofibrations.

What is now left is to show that is a cofibration. This will follow from the next proposition. With it, we will have completed the proof that a relative CW complex is a cofibration.

Proposition 7Suppose is the union of a sequence of subspaces with the weak topology. If the maps are cofibrations, then is a cofibration.

*Proof:* Indeed, let us verify that the pair satisfies the homotopy extension property. Suppose we have a map and a homotopy . Then the map is equivalent to a sequence of continuous maps . Suppose inductively we have constructed extending . Then we can use the HEP to extend to . Repeating this inductively, we get a compatible set of homotopies which agree with on . Gluing them together (note that has the weak topology) gives the extension of the initial homotopy.

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