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 1
is 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 2 Consider 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 3 Suppose
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 4 Let
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 5 Let
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 6 Let
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 7 Suppose
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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