4.8 The object classifier
In type theory we have a basic notion of family of types, namely a function .We have seen that such families behave somewhat like fibrations
in homotopy theory, with the fibration being the projection .A basic fact in homotopy theory is that every map is equivalent
to a fibration.With univalence at our disposal, we can prove the same thing in type theory.
Lemma 4.8.1.
For any type family , the fiber of over is equivalent to :
Proof.
We have
using the left universal property of identity types.∎
Lemma 4.8.2.
For any function , we have .
Proof.
We have
using the fact that is contractible.∎
Theorem 4.8.3.
For any type there is an equivalence
Proof.
We have to construct quasi-inverses
We define by , and by .Now we have to verify that and that .
- 1.
Let .By Lemma 4.8.1 (http://planetmath.org/48theobjectclassifier#Thmprelem1), for any , so it follows immediatelythat .
- 2.
Let be a function. We have to find a path
First note that by Lemma 4.8.2 (http://planetmath.org/48theobjectclassifier#Thmprelem2), we have with and .By Theorem 2.7.2 (http://planetmath.org/27sigmatypes#Thmprethm1), it remains to show .But by the computation rule for univalence and (2.9.4) (http://planetmath.org/29pitypesandthefunctionextensionalityaxiom#S0.E3), we have , and the definition of immediately yields .∎
In particular, this implies that we have an object classifier in the sense of higher topos theory.Recall from Definition 2.1.7 (http://planetmath.org/21typesarehighergroupoids#Thmpredefn1) that denotes the type of pointed types.
Theorem 4.8.4.
Let be a function. Then the diagram
HoTT_fig_4.8.1.png
is a pullback square (see http://planetmath.org/node/87642Exercise 2.11).Here the function is defined by
Proof.
Note that we have the equivalences
which gives us a composite equivalence .We may display the action of this composite equivalence step by step by
Therefore, we get homotopies and .∎