Algebraic Topology Functors

This will be old news to anyone who does algebraic topology, but oddly enough I can’t seem to find it all written in one place anywhere, and in particular I can’t find the bit about {\mathsf{hPairTop}} at all.

In algebraic topology you (for example) associate every topological space {X} with a group, like {\pi_1(X, x_0)} or {H_5(X)}. All of these operations turn out to be functors. This isn’t surprising, because as far as I’m concerned the definition of a functor is “any time you take one type of object and naturally make another object”.

The surprise is that these objects also respect homotopy in a nice way; proving this is a fair amount of the “setup” work in algebraic topology.

1. Homology, {H_n : \mathsf{hTop} \rightarrow \mathsf{Grp}}

Note that {H_5} is a functor

\displaystyle H_5 : \mathsf{Top} \rightarrow \mathsf{Grp}

i.e. to every space {X} we can associate a group {H_5(X)}. (Of course, replace {5} by integer of your choice.) Recall that:

Definition 1

Two maps {f, g : X \rightarrow Y} are homotopy equivalent if there exists a homotopy between them.

Thus for a map we can take its homotopy class {[f]} (the equivalence class under this relationship). This has the nice property that {[f \circ g] = [f] \circ [g]} and so on.

Definition 2

Two spaces {X} and {Y} are homotopic if there exists a pair of maps {f : X \rightarrow Y} and {g : Y \rightarrow X} such that {[f \circ g] = [\mathrm{id}_X]} and {[g \circ f] = [\mathrm{id}_Y]}.

In light of this, we can define

Definition 3

The category {\mathsf{hTop}} is defined as follows:

  • The objects are topological spaces {X}.
  • The morphisms {X \rightarrow Y} are homotopy classes of continuous maps {X \rightarrow Y}.

Remark 4

Composition is well-defined since {[f \circ g] = [f] \circ [g]}. Two spaces are isomorphic in {\mathsf{hTop}} if they are homotopic.

Remark 5

As you might guess this “quotient” construction is called a quotient category.

Then the big result is that:

Theorem 6

The induced map {f_\sharp = H_n(f)} of a map {f: X \rightarrow Y} depends only on the homotopy class of {f}. Thus {H_n} is a functor

\displaystyle H_n : \mathsf{hTop} \rightarrow \mathsf{Grp}.

The proof of this is geometric, using the so-called prism operators. In any case, as with all functors we deduce

Corollary 7

{H_n(X) \cong H_n(Y)} if {X} and {Y} are homotopic.

In particular, the contractible spaces are those spaces {X} which are homotopy equivalent to a point. In which case, {H_n(X) = 0} for all {n \ge 1}.

2. Relative Homology, {H_n : \mathsf{hPairTop} \rightarrow \mathsf{Grp}}

In fact, we also defined homology groups

\displaystyle H_n(X,A)

for {A \subseteq X}. We will now show this is functorial too.

Definition 8

Let {\varnothing \neq A \subset X} and {\varnothing \neq B \subset X} be subspaces, and consider a map {f : X \rightarrow Y}. If {f(A) \subseteq B} we write

\displaystyle f : (X,A) \rightarrow (Y,B).

We say {f} is a map of pairs, between the pairs {(X,A)} and {(Y,B)}.

Definition 9

We say that {f,g : (X,A) \rightarrow (Y,B)} are pair-homotopic if they are “homotopic through maps of pairs”.

More formally, a pair-homotopy {f, g : (X,A) \rightarrow (Y,B)} is a map {F : [0,1] \times X \rightarrow Y}, which we’ll write as {F_t(X)}, such that {F} is a homotopy of the maps {f,g : X \rightarrow Y} and each {F_t} is itself a map of pairs.

Thus, we naturally arrive at two categories:

  • {\mathsf{PairTop}}, the category of pairs of toplogical spaces, and
  • {\mathsf{hPairTop}}, the same category except with maps only equivalent up to homotopy.

Definition 10

As before, we say pairs {(X,A)} and {(Y,B)} are pair-homotopy equivalent if they are isomorphic in {\mathsf{hPairTop}}. An isomorphism of {\mathsf{hPairTop}} is a pair-homotopy equivalence.

Then, the prism operators now let us derive

Theorem 11

We have a functor

\displaystyle H_n : \mathsf{hPairTop} \rightarrow \mathsf{Grp}.

The usual corollaries apply.

Now, we want an analog of contractible spaces for our pairs: i.e. pairs of spaces {(X,A)} such that {H_n(X,A) = 0} for {n \ge 1}. The correct definition is:

Definition 12

Let {A \subset X}. We say that {A} is a deformation retract of {X} if there is a map of pairs {r : (X, A) \rightarrow (A, A)} which is a pair homotopy equivalence.

Example 13 (Examples of Deformation Retracts)

  1. If a single point {p} is a deformation retract of a space {X}, then {X} is contractible, since the retraction {r : X \rightarrow \{\ast\}} (when viewed as a map {X \rightarrow X}) is homotopic to the identity map {\mathrm{id}_X : X \rightarrow X}.
  2. The punctured disk {D^2 \setminus \{0\}} deformation retracts onto its boundary {S^1}.
  3. More generally, {D^{n} \setminus \{0\}} deformation retracts onto its boundary {S^{n-1}}.
  4. Similarly, {\mathbb R^n \setminus \{0\}} deformation retracts onto a sphere {S^{n-1}}.

Of course in this situation we have that

\displaystyle H_n(X,A) \cong H_n(A,A) = 0.

3. Homotopy, {\pi_1 : \mathsf{hTop}_\ast \rightarrow \mathsf{Grp}}

As a special case of the above, we define

Definition 14

The category {\mathsf{Top}_\ast} is defined as follows:

  • The objects are pairs {(X, x_0)} of spaces {X} with a distinguished basepoint {x_0}. We call these pointed spaces.
  • The morphisms are maps {f : (X, x_0) \rightarrow (Y, y_0)}, meaning {f} is continuous and {f(x_0) = y_0}.

Now again we mod out:

Definition 15

Two maps {f , g : (X, x_0) \rightarrow (Y, y_0)} of pointed spaces are homotopic if there is a homotopy between them which also fixes the basepoints. We can then, in the same way as before, define the quotient category {\mathsf{hTop}_\ast}.

And lo and behold:

Theorem 16

We have a functor

\displaystyle \pi_1 : \mathsf{hTop}_\ast \rightarrow \mathsf{Grp}.

Same corollaries as before.

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