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单词 48TheObjectClassifier
释义

4.8 The object classifier


In type theoryPlanetmathPlanetmath we have a basic notion of family of types, namely a function B:A𝒰.We have seen that such families behave somewhat like fibrationsMathworldPlanetmath in homotopy theory, with the fibration being the projection 𝗉𝗋1:(a:A)B(a)A.A basic fact in homotopy theory is that every map is equivalentMathworldPlanetmathPlanetmathPlanetmathPlanetmath 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 B:AU, the fiber of pr1:(x:A)B(x)A over a:A is equivalent to B(a):

𝖿𝗂𝖻𝗉𝗋1(a)B(a)
Proof.

We have

𝖿𝗂𝖻𝗉𝗋1(a):u:(x:A)B(x)𝗉𝗋1(u)=a
(x:A)(b:B(x))(x=a)
(x:A)(p:x=a)B(x)
B(a)

using the left universal propertyMathworldPlanetmath of identity types.∎

Lemma 4.8.2.

For any function f:AB, we have A(b:B)fibf(b).

Proof.

We have

b:B𝖿𝗂𝖻f(b):(b:B)(a:A)(f(a)=b)
(a:A)(b:B)(f(a)=b)
A

using the fact that (b:B)(f(a)=b) is contractibleMathworldPlanetmath.∎

Theorem 4.8.3.

For any type B there is an equivalence

χ:(A:𝒰(AB))(B𝒰).
Proof.

We have to construct quasi-inversesPlanetmathPlanetmath

χ:(A:𝒰(AB))B𝒰
ψ:(B𝒰)(A:𝒰(AB)).

We define χ by χ((A,f),b):𝖿𝗂𝖻f(b), and ψ by ψ(P):(((b:B)P(b)),𝗉𝗋1).Now we have to verify that χψ𝗂𝖽 and that ψχ𝗂𝖽.

  1. 1.

    Let P:B𝒰.By Lemma 4.8.1 (http://planetmath.org/48theobjectclassifier#Thmprelem1),𝖿𝗂𝖻𝗉𝗋1(b)P(b) for any b:B, so it follows immediatelythat Pχ(ψ(P)).

  2. 2.

    Let f:AB be a function. We have to find a path

    ((b:B)𝖿𝗂𝖻f(b),𝗉𝗋1)=(A,f).

    First note that by Lemma 4.8.2 (http://planetmath.org/48theobjectclassifier#Thmprelem2), we havee:(b:B)𝖿𝗂𝖻f(b)A with e(b,a,p):a and e-1(a):(f(a),a,𝗋𝖾𝖿𝗅f(a)).By Theorem 2.7.2 (http://planetmath.org/27sigmatypes#Thmprethm1), it remains to show (𝗎𝖺(e))*(𝗉𝗋1)=f.But by the computation rule for univalence and (2.9.4) (http://planetmath.org/29pitypesandthefunctionextensionalityaxiom#S0.E3), we have (𝗎𝖺(e))*(𝗉𝗋1)=𝗉𝗋1e-1, and the definition of e-1 immediately yields 𝗉𝗋1e-1f.∎

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 𝒰 denotes the type (A:𝒰)A of pointed types.

Theorem 4.8.4.

Let f:AB be a function. Then the diagram

\\includegraphics

HoTT_fig_4.8.1.png

is a pullback square (see http://planetmath.org/node/87642Exercise 2.11).Here the function ϑf is defined by

λa.(𝖿𝗂𝖻f(f(a)),(a,𝗋𝖾𝖿𝗅f(a))).
Proof.

Note that we have the equivalences

Ab:B𝖿𝗂𝖻f(b)
(b:B)(X:𝒰)(p:𝖿𝗂𝖻f(b)=X)X
(b:B)(X:𝒰)(x:X)𝖿𝗂𝖻f(b)=X
(b:B)(Y:𝒰)𝖿𝗂𝖻f(b)=𝗉𝗋1Y
B×𝒰𝒰.

which gives us a composite equivalence e:AB×𝒰𝒰.We may display the action of this composite equivalence step by step by

a(f(a),(a,𝗋𝖾𝖿𝗅f(a)))
(f(a),𝖿𝗂𝖻f(f(a)),𝗋𝖾𝖿𝗅𝖿𝗂𝖻f(f(a)),(a,𝗋𝖾𝖿𝗅f(a)))
(f(a),𝖿𝗂𝖻f(f(a)),(a,𝗋𝖾𝖿𝗅f(a)),𝗋𝖾𝖿𝗅𝖿𝗂𝖻f(f(a))).

Therefore, we get homotopiesMathworldPlanetmath f𝗉𝗋1e and ϑf𝗉𝗋2e.∎

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