The univalence axiom

Content created by Fredrik Bakke, Egbert Rijke, Jonathan Prieto-Cubides, Eléonore Mangel, Elisabeth Stenholm, Raymond Baker and Vojtěch Štěpančík.

Created on 2022-02-15.
Last modified on 2024-04-25.

module foundation.univalence where

open import foundation-core.univalence public
open import foundation.action-on-identifications-functions
open import foundation.dependent-pair-types
open import foundation.equality-dependent-function-types
open import foundation.equivalences
open import foundation.fundamental-theorem-of-identity-types
open import foundation.universe-levels

open import foundation-core.coherently-invertible-maps
open import foundation-core.contractible-types
open import foundation-core.function-types
open import foundation-core.functoriality-dependent-pair-types
open import foundation-core.homotopies
open import foundation-core.identity-types
open import foundation-core.injective-maps
open import foundation-core.retractions
open import foundation-core.sections
open import foundation-core.torsorial-type-families


The univalence axiom characterizes the identity types of universes. It asserts that the map (A = B) → (A ≃ B) is an equivalence.

In this file we postulate the univalence axiom. Its statement is defined in foundation-core.univalence.


Rather than postulating a witness of univalence-axiom directly, we postulate the constituents of a coherent two-sided inverse to equiv-eq. The benefits are that we end up with a single converse map to equiv-eq, rather than a separate section and retraction, although they would be homotopic regardless. In addition, this formulation helps Agda display goals involving the univalence axiom in a more readable way.

module _
  {l : Level} {A B : UU l}

    eq-equiv : A  B  A  B

    is-section-eq-equiv : is-section equiv-eq eq-equiv

    is-retraction-eq-equiv' : is-retraction equiv-eq eq-equiv

    coh-eq-equiv' :
        ( equiv-eq)
        ( eq-equiv)
        ( is-section-eq-equiv)
        ( is-retraction-eq-equiv')

univalence : univalence-axiom
univalence A B =
  is-equiv-is-invertible eq-equiv is-section-eq-equiv is-retraction-eq-equiv'


module _
  {l : Level} {A B : UU l}

  equiv-univalence : (A  B)  (A  B)
  pr1 equiv-univalence = equiv-eq
  pr2 equiv-univalence = univalence A B

    is-retraction-eq-equiv : is-retraction (equiv-eq {A = A} {B}) eq-equiv
    is-retraction-eq-equiv =
      is-retraction-map-inv-is-equiv (univalence A B)

module _
  {l : Level}

  is-equiv-eq-equiv : (A B : UU l)  is-equiv (eq-equiv {A = A} {B})
  is-equiv-eq-equiv A B =
    is-equiv-is-invertible equiv-eq is-retraction-eq-equiv' is-section-eq-equiv

  compute-eq-equiv-id-equiv : (A : UU l)  eq-equiv {A = A} id-equiv  refl
  compute-eq-equiv-id-equiv A = is-retraction-eq-equiv refl

  equiv-eq-equiv : (A B : UU l)  (A  B)  (A  B)
  pr1 (equiv-eq-equiv A B) = eq-equiv
  pr2 (equiv-eq-equiv A B) = is-equiv-eq-equiv A B

The total space of all equivalences out of a type or into a type is contractible

Type families of which the total space is contractible are also called torsorial. This terminology originates from higher group theory, where a higher group action is torsorial if its type of orbits, i.e., its total space, is contractible. Our claim that the total space of all equivalences out of a type A is contractible can therefore be stated more succinctly as the claim that the family of equivalences out of A is torsorial.

module _
  {l : Level}

    is-torsorial-equiv :
      (A : UU l)  is-torsorial  (X : UU l)  A  X)
    is-torsorial-equiv A =
      is-torsorial-equiv-based-univalence A (univalence A)

    is-torsorial-equiv' :
      (A : UU l)  is-torsorial  (X : UU l)  X  A)
    is-torsorial-equiv' A =
        ( Σ (UU l)  X  X  A))
        ( equiv-tot  X  equiv-univalence))
        ( is-torsorial-Id' A)

Univalence for type families

equiv-fam :
  {l1 l2 l3 : Level} {A : UU l1} (B : A  UU l2) (C : A  UU l3) 
  UU (l1  l2  l3)
equiv-fam {A = A} B C = (a : A)  B a  C a

id-equiv-fam :
  {l1 l2 : Level} {A : UU l1} (B : A  UU l2)  equiv-fam B B
id-equiv-fam B a = id-equiv

equiv-eq-fam :
  {l1 l2 : Level} {A : UU l1} (B C : A  UU l2)  B  C  equiv-fam B C
equiv-eq-fam B .B refl = id-equiv-fam B

  is-torsorial-equiv-fam :
    {l1 l2 : Level} {A : UU l1} (B : A  UU l2) 
    is-torsorial  (C : A  UU l2)  equiv-fam B C)
  is-torsorial-equiv-fam B =
    is-torsorial-Eq-Π  x  is-torsorial-equiv (B x))

  is-equiv-equiv-eq-fam :
    {l1 l2 : Level} {A : UU l1} (B C : A  UU l2)  is-equiv (equiv-eq-fam B C)
  is-equiv-equiv-eq-fam B =
      ( is-torsorial-equiv-fam B)
      ( equiv-eq-fam B)

extensionality-fam :
  {l1 l2 : Level} {A : UU l1} (B C : A  UU l2)  (B  C)  equiv-fam B C
pr1 (extensionality-fam B C) = equiv-eq-fam B C
pr2 (extensionality-fam B C) = is-equiv-equiv-eq-fam B C

eq-equiv-fam :
  {l1 l2 : Level} {A : UU l1} {B C : A  UU l2}  equiv-fam B C  B  C
eq-equiv-fam {B = B} {C} = map-inv-is-equiv (is-equiv-equiv-eq-fam B C)

Computations with univalence

compute-equiv-eq-concat :
  {l : Level} {A B C : UU l} (p : A  B) (q : B  C) 
  equiv-eq q ∘e equiv-eq p  equiv-eq (p  q)
compute-equiv-eq-concat refl refl = eq-equiv-eq-map-equiv refl

compute-eq-equiv-comp-equiv :
  {l : Level} {A B C : UU l} (f : A  B) (g : B  C) 
  eq-equiv f  eq-equiv g  eq-equiv (g ∘e f)
compute-eq-equiv-comp-equiv f g =
    ( equiv-univalence)
    ( ( inv ( compute-equiv-eq-concat (eq-equiv f) (eq-equiv g))) 
      ( ( ap
          ( λ e  (map-equiv e g) ∘e (equiv-eq (eq-equiv f)))
          ( right-inverse-law-equiv equiv-univalence)) 
        ( ( ap
            ( λ e  g ∘e map-equiv e f)
            ( right-inverse-law-equiv equiv-univalence)) 
          ( ap
            ( λ e  map-equiv e (g ∘e f))
            ( inv (right-inverse-law-equiv equiv-univalence))))))

compute-map-eq-ap-inv :
  {l1 l2 : Level} {A : UU l1} {B : A  UU l2} {x y : A} (p : x  y) 
  map-eq (ap B (inv p))  map-eq (ap B p) ~ id
compute-map-eq-ap-inv refl = refl-htpy

commutativity-inv-equiv-eq :
  {l : Level} {A B : UU l} (p : A  B) 
  inv-equiv (equiv-eq p)  equiv-eq (inv p)
commutativity-inv-equiv-eq refl = eq-equiv-eq-map-equiv refl

commutativity-inv-eq-equiv :
  {l : Level} {A B : UU l} (f : A  B) 
  inv (eq-equiv f)  eq-equiv (inv-equiv f)
commutativity-inv-eq-equiv f =
    ( equiv-univalence)
    ( ( inv (commutativity-inv-equiv-eq (eq-equiv f))) 
      ( ( ap
          ( λ e  (inv-equiv (map-equiv e f)))
          ( right-inverse-law-equiv equiv-univalence)) 
        ( ap
          ( λ e  map-equiv e (inv-equiv f))
          ( inv (right-inverse-law-equiv equiv-univalence)))))

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