Transposing identifications along retractions

Content created by Egbert Rijke and maybemabeline.

Created on 2024-02-23.
Last modified on 2024-02-23.

module foundation.transposition-identifications-along-retractions where
open import foundation.action-on-identifications-functions
open import foundation.dependent-pair-types
open import foundation.function-extensionality
open import foundation.universe-levels

open import foundation-core.equivalences
open import foundation-core.function-types
open import foundation-core.homotopies
open import foundation-core.identity-types
open import foundation-core.retractions


Consider a map f : A → B and a map g : B → A in the converse direction. Then there is an equivalence

  is-retraction f g ≃ ((x : A) (y : B) → (f x = y) ≃ (x = g y))

In other words, any retracting homotopy g ∘ f ~ id induces a unique family of transposition maps

  f x = y → x = g y

indexed by x : A and y : B.


Transposing identifications along retractions

module _
  {l1 l2 : Level} {A : UU l1} {B : UU l2} (f : A  B) (g : B  A)

  eq-transpose-is-retraction :
    is-retraction f g  {x : B} {y : A}  x  f y  g x  y
  eq-transpose-is-retraction H {x} {y} p = ap g p  H y

  eq-transpose-is-retraction' :
    is-retraction f g  {x : A} {y : B}  f x  y  x  g y
  eq-transpose-is-retraction' H {x} refl = inv (H x)


The map that assings to each retracting homotopy a family of transposition functions of identifications is an equivalence

module _
  {l1 l2 : Level} {A : UU l1} {B : UU l2} (f : A  B) (g : B  A)

  is-retraction-eq-transpose :
    ({x : B} {y : A}  x  f y  g x  y)  is-retraction f g
  is-retraction-eq-transpose H x = H refl

  is-retraction-eq-transpose' :
    ({x : A} {y : B}  f x  y  x  g y)  is-retraction f g
  is-retraction-eq-transpose' H x = inv (H refl)

  is-retraction-is-retraction-eq-transpose :
    is-retraction-eq-transpose  eq-transpose-is-retraction f g ~ id
  is-retraction-is-retraction-eq-transpose H = refl

  htpy-is-section-is-retraction-eq-transpose :
    (H : {x : B} {y : A}  x  f y  g x  y)
    (x : B) (y : A) 
    eq-transpose-is-retraction f g (is-retraction-eq-transpose H) {x} {y} ~
    H {x} {y}
  htpy-is-section-is-retraction-eq-transpose H x y refl = refl

    is-section-is-retraction-eq-transpose :
      eq-transpose-is-retraction f g  is-retraction-eq-transpose ~ id
    is-section-is-retraction-eq-transpose H =
        ( λ x 
            ( λ y  eq-htpy (htpy-is-section-is-retraction-eq-transpose H x y)))

  is-equiv-eq-transpose-is-retraction :
    is-equiv (eq-transpose-is-retraction f g)
  is-equiv-eq-transpose-is-retraction =
      ( is-retraction-eq-transpose)
      ( is-section-is-retraction-eq-transpose)
      ( is-retraction-is-retraction-eq-transpose)

  equiv-eq-transpose-is-retraction :
    is-retraction f g  ({x : B} {y : A}  x  f y  g x  y)
  equiv-eq-transpose-is-retraction =
    ( eq-transpose-is-retraction f g , is-equiv-eq-transpose-is-retraction)

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