Functoriality of dependent pair types

Content created by Fredrik Bakke, Egbert Rijke, Jonathan Prieto-Cubides, Raymond Baker and Victor Blanchi.

Created on 2022-02-07.
Last modified on 2024-03-01.

module foundation-core.functoriality-dependent-pair-types where
Imports 
open import foundation.dependent-pair-types
open import foundation.universe-levels

open import foundation-core.contractible-maps
open import foundation-core.contractible-types
open import foundation-core.equality-dependent-pair-types
open import foundation-core.equivalences
open import foundation-core.families-of-equivalences
open import foundation-core.fibers-of-maps
open import foundation-core.function-types
open import foundation-core.homotopies
open import foundation-core.identity-types
open import foundation-core.transport-along-identifications

Idea

Any map f : A → B and any family of maps g : (a : A) → C a → D (f a) together induce a map map-Σ D f g : Σ A C → Σ B D. Specific instances of this construction are also very useful: if we take f to be the identity map, then we see that any family of maps g : (a : A) → C a → D a induces a map on total spaces Σ A C → Σ A D; if we take g to be the family of identity maps, then we see that for any family D over B, any map f : A → B induces a map Σ A (D ∘ f) → Σ B D.

Definitions

The induced map on total spaces

module _
  {l1 l2 l3 : Level} {A : UU l1} {B : A → UU l2} {C : A → UU l3}
  (f : (x : A) → B x → C x)
  where

Any family of maps induces a map on the total spaces.

  tot : Σ A B → Σ A C
  pr1 (tot t) = pr1 t
  pr2 (tot t) = f (pr1 t) (pr2 t)

Any map f : A → B induces a map Σ A (C ∘ f) → Σ B C

module _
  {l1 l2 l3 : Level} {A : UU l1} {B : UU l2} (f : A → B) (C : B → UU l3)
  where

  map-Σ-map-base : Σ A (λ x → C (f x)) → Σ B C
  pr1 (map-Σ-map-base s) = f (pr1 s)
  pr2 (map-Σ-map-base s) = pr2 s

The functorial action of dependent pair types, and its defining homotopy

module _
  {l1 l2 l3 l4 : Level} {A : UU l1} {B : UU l2} {C : A → UU l3}
  (D : B → UU l4)
  where

  map-Σ :
    (f : A → B) (g : (x : A) → C x → D (f x)) → Σ A C → Σ B D
  pr1 (map-Σ f g t) = f (pr1 t)
  pr2 (map-Σ f g t) = g (pr1 t) (pr2 t)

  triangle-map-Σ :
    (f : A → B) (g : (x : A) → C x → D (f x)) →
    (map-Σ f g) ~ (map-Σ-map-base f D ∘ tot g)
  triangle-map-Σ f g t = refl

Properties

The map map-Σ preserves homotopies

module _
  {l1 l2 l3 l4 : Level} {A : UU l1} {B : UU l2} {C : A → UU l3}
  (D : B → UU l4)
  where

  htpy-map-Σ :
    {f f' : A → B} (H : f ~ f')
    (g : (x : A) → C x → D (f x)) {g' : (x : A) → C x → D (f' x)}
    (K : (x : A) → ((tr D (H x)) ∘ (g x)) ~ (g' x)) →
    (map-Σ D f g) ~ (map-Σ D f' g')
  htpy-map-Σ H g K t = eq-pair-Σ' (pair (H (pr1 t)) (K (pr1 t) (pr2 t)))

The map tot preserves homotopies

tot-htpy :
  {l1 l2 l3 : Level} {A : UU l1} {B : A → UU l2} {C : A → UU l3}
  {f g : (x : A) → B x → C x} → (H : (x : A) → f x ~ g x) → tot f ~ tot g
tot-htpy H (pair x y) = eq-pair-eq-fiber (H x y)

The map tot preserves identity maps

tot-id :
  {l1 l2 : Level} {A : UU l1} (B : A → UU l2) →
  (tot (λ x (y : B x) → y)) ~ id
tot-id B (pair x y) = refl

The map tot preserves composition

preserves-comp-tot :
  {l1 l2 l3 l4 : Level}
  {A : UU l1} {B : A → UU l2} {B' : A → UU l3} {B'' : A → UU l4}
  (f : (x : A) → B x → B' x) (g : (x : A) → B' x → B'' x) →
  tot (λ x → (g x) ∘ (f x)) ~ ((tot g) ∘ (tot f))
preserves-comp-tot f g (pair x y) = refl

The fibers of tot

We show that for any family of maps, the fiber of the induced map on total spaces are equivalent to the fibers of the maps in the family.

module _
  {l1 l2 l3 : Level} {A : UU l1} {B : A → UU l2} {C : A → UU l3}
  (f : (x : A) → B x → C x)
  where

  map-compute-fiber-tot :
    (t : Σ A C) → fiber (tot f) t → fiber (f (pr1 t)) (pr2 t)
  pr1 (map-compute-fiber-tot .(tot f (pair x y)) (pair (pair x y) refl)) = y
  pr2 (map-compute-fiber-tot .(tot f (pair x y)) (pair (pair x y) refl)) = refl

  map-inv-compute-fiber-tot :
    (t : Σ A C) → fiber (f (pr1 t)) (pr2 t) → fiber (tot f) t
  pr1 (pr1 (map-inv-compute-fiber-tot (pair a .(f a y)) (pair y refl))) = a
  pr2 (pr1 (map-inv-compute-fiber-tot (pair a .(f a y)) (pair y refl))) = y
  pr2 (map-inv-compute-fiber-tot (pair a .(f a y)) (pair y refl)) = refl

  is-section-map-inv-compute-fiber-tot :
    (t : Σ A C) → (map-compute-fiber-tot t ∘ map-inv-compute-fiber-tot t) ~ id
  is-section-map-inv-compute-fiber-tot (pair x .(f x y)) (pair y refl) = refl

  is-retraction-map-inv-compute-fiber-tot :
    (t : Σ A C) → (map-inv-compute-fiber-tot t ∘ map-compute-fiber-tot t) ~ id
  is-retraction-map-inv-compute-fiber-tot ._ (pair (pair x y) refl) =
    refl

  abstract
    is-equiv-map-compute-fiber-tot :
      (t : Σ A C) → is-equiv (map-compute-fiber-tot t)
    is-equiv-map-compute-fiber-tot t =
      is-equiv-is-invertible
        ( map-inv-compute-fiber-tot t)
        ( is-section-map-inv-compute-fiber-tot t)
        ( is-retraction-map-inv-compute-fiber-tot t)

  compute-fiber-tot : (t : Σ A C) → fiber (tot f) t ≃ fiber (f (pr1 t)) (pr2 t)
  pr1 (compute-fiber-tot t) = map-compute-fiber-tot t
  pr2 (compute-fiber-tot t) = is-equiv-map-compute-fiber-tot t

  abstract
    is-equiv-map-inv-compute-fiber-tot :
      (t : Σ A C) → is-equiv (map-inv-compute-fiber-tot t)
    is-equiv-map-inv-compute-fiber-tot t =
      is-equiv-is-invertible
        ( map-compute-fiber-tot t)
        ( is-retraction-map-inv-compute-fiber-tot t)
        ( is-section-map-inv-compute-fiber-tot t)

  inv-compute-fiber-tot :
    (t : Σ A C) → fiber (f (pr1 t)) (pr2 t) ≃ fiber (tot f) t
  pr1 (inv-compute-fiber-tot t) = map-inv-compute-fiber-tot t
  pr2 (inv-compute-fiber-tot t) = is-equiv-map-inv-compute-fiber-tot t

A family of maps f is a family of equivalences if and only if tot f is an equivalence

module _
  {l1 l2 l3 : Level} {A : UU l1} {B : A → UU l2} {C : A → UU l3}
  {f : (x : A) → B x → C x}
  where

  abstract
    is-equiv-tot-is-fiberwise-equiv : is-fiberwise-equiv f → is-equiv (tot f)
    is-equiv-tot-is-fiberwise-equiv H =
      is-equiv-is-contr-map
        ( λ t →
          is-contr-is-equiv
            ( fiber (f (pr1 t)) (pr2 t))
            ( map-compute-fiber-tot f t)
            ( is-equiv-map-compute-fiber-tot f t)
            ( is-contr-map-is-equiv (H (pr1 t)) (pr2 t)))

  abstract
    is-fiberwise-equiv-is-equiv-tot : is-equiv (tot f) → is-fiberwise-equiv f
    is-fiberwise-equiv-is-equiv-tot is-equiv-tot-f x =
      is-equiv-is-contr-map
        ( λ z →
          is-contr-is-equiv'
            ( fiber (tot f) (pair x z))
            ( map-compute-fiber-tot f (pair x z))
            ( is-equiv-map-compute-fiber-tot f (pair x z))
            ( is-contr-map-is-equiv is-equiv-tot-f (pair x z)))

The action of tot on equivalences

module _
  {l1 l2 l3 : Level} {A : UU l1} {B : A → UU l2} {C : A → UU l3}
  where

  equiv-tot : ((x : A) → B x ≃ C x) → (Σ A B) ≃ (Σ A C)
  pr1 (equiv-tot e) = tot (λ x → map-equiv (e x))
  pr2 (equiv-tot e) =
    is-equiv-tot-is-fiberwise-equiv (λ x → is-equiv-map-equiv (e x))

The fibers of map-Σ-map-base

module _
  {l1 l2 l3 : Level} {A : UU l1} {B : UU l2} (f : A → B) (C : B → UU l3)
  where

  fiber-map-Σ-map-base-fiber :
    (t : Σ B C) → fiber f (pr1 t) → fiber (map-Σ-map-base f C) t
  pr1 (pr1 (fiber-map-Σ-map-base-fiber (pair .(f x) z) (pair x refl))) = x
  pr2 (pr1 (fiber-map-Σ-map-base-fiber (pair .(f x) z) (pair x refl))) = z
  pr2 (fiber-map-Σ-map-base-fiber (pair .(f x) z) (pair x refl)) = refl

  fiber-fiber-map-Σ-map-base :
    (t : Σ B C) → fiber (map-Σ-map-base f C) t → fiber f (pr1 t)
  pr1
    ( fiber-fiber-map-Σ-map-base
      .(map-Σ-map-base f C (pair x z)) (pair (pair x z) refl)) = x
  pr2
    ( fiber-fiber-map-Σ-map-base
      .(map-Σ-map-base f C (pair x z)) (pair (pair x z) refl)) = refl

  is-section-fiber-fiber-map-Σ-map-base :
    (t : Σ B C) →
    (fiber-map-Σ-map-base-fiber t ∘ fiber-fiber-map-Σ-map-base t) ~ id
  is-section-fiber-fiber-map-Σ-map-base .(pair (f x) z) (pair (pair x z) refl) =
    refl

  is-retraction-fiber-fiber-map-Σ-map-base :
    (t : Σ B C) →
    (fiber-fiber-map-Σ-map-base t ∘ fiber-map-Σ-map-base-fiber t) ~ id
  is-retraction-fiber-fiber-map-Σ-map-base (pair .(f x) z) (pair x refl) = refl

  abstract
    is-equiv-fiber-map-Σ-map-base-fiber :
      (t : Σ B C) → is-equiv (fiber-map-Σ-map-base-fiber t)
    is-equiv-fiber-map-Σ-map-base-fiber t =
      is-equiv-is-invertible
        ( fiber-fiber-map-Σ-map-base t)
        ( is-section-fiber-fiber-map-Σ-map-base t)
        ( is-retraction-fiber-fiber-map-Σ-map-base t)

  equiv-fiber-map-Σ-map-base-fiber :
    (t : Σ B C) → fiber f (pr1 t) ≃ fiber (map-Σ-map-base f C) t
  pr1 (equiv-fiber-map-Σ-map-base-fiber t) = fiber-map-Σ-map-base-fiber t
  pr2 (equiv-fiber-map-Σ-map-base-fiber t) =
    is-equiv-fiber-map-Σ-map-base-fiber t

If f : A → B is a contractible map, then so is map-Σ-map-base f C

module _
  {l1 l2 l3 : Level} {A : UU l1} {B : UU l2} (f : A → B) (C : B → UU l3)
  where

  abstract
    is-contr-map-map-Σ-map-base :
      is-contr-map f → is-contr-map (map-Σ-map-base f C)
    is-contr-map-map-Σ-map-base is-contr-f (pair y z) =
      is-contr-equiv'
        ( fiber f y)
        ( equiv-fiber-map-Σ-map-base-fiber f C (pair y z))
        ( is-contr-f y)

If f : A → B is an equivalence, then so is map-Σ-map-base f C

module _
  {l1 l2 l3 : Level} {A : UU l1} {B : UU l2} (f : A → B) (C : B → UU l3)
  where

  abstract
    is-equiv-map-Σ-map-base : is-equiv f → is-equiv (map-Σ-map-base f C)
    is-equiv-map-Σ-map-base is-equiv-f =
      is-equiv-is-contr-map
        ( is-contr-map-map-Σ-map-base f C (is-contr-map-is-equiv is-equiv-f))

equiv-Σ-equiv-base :
  {l1 l2 l3 : Level} {A : UU l1} {B : UU l2} (C : B → UU l3) (e : A ≃ B) →
  Σ A (C ∘ map-equiv e) ≃ Σ B C
pr1 (equiv-Σ-equiv-base C (pair f is-equiv-f)) = map-Σ-map-base f C
pr2 (equiv-Σ-equiv-base C (pair f is-equiv-f)) =
  is-equiv-map-Σ-map-base f C is-equiv-f

The functorial action of dependent pair types preserves equivalences

module _
  {l1 l2 l3 l4 : Level} {A : UU l1} {B : UU l2} {C : A → UU l3}
  (D : B → UU l4)
  where

  abstract
    is-equiv-map-Σ :
      {f : A → B} {g : (x : A) → C x → D (f x)} →
      is-equiv f → is-fiberwise-equiv g → is-equiv (map-Σ D f g)
    is-equiv-map-Σ {f} {g} is-equiv-f is-fiberwise-equiv-g =
      is-equiv-left-map-triangle
        ( map-Σ D f g)
        ( map-Σ-map-base f D)
        ( tot g)
        ( triangle-map-Σ D f g)
        ( is-equiv-tot-is-fiberwise-equiv is-fiberwise-equiv-g)
        ( is-equiv-map-Σ-map-base f D is-equiv-f)

  equiv-Σ :
    (e : A ≃ B) (g : (x : A) → C x ≃ D (map-equiv e x)) → Σ A C ≃ Σ B D
  pr1 (equiv-Σ e g) = map-Σ D (map-equiv e) (λ x → map-equiv (g x))
  pr2 (equiv-Σ e g) =
    is-equiv-map-Σ
      ( is-equiv-map-equiv e)
      ( λ x → is-equiv-map-equiv (g x))

  abstract
    is-fiberwise-equiv-is-equiv-map-Σ :
      (f : A → B) (g : (x : A) → C x → D (f x)) →
      is-equiv f → is-equiv (map-Σ D f g) → is-fiberwise-equiv g
    is-fiberwise-equiv-is-equiv-map-Σ f g H K =
      is-fiberwise-equiv-is-equiv-tot
        ( is-equiv-top-map-triangle
          ( map-Σ D f g)
          ( map-Σ-map-base f D)
          ( tot g)
          ( triangle-map-Σ D f g)
          ( is-equiv-map-Σ-map-base f D H)
          ( K))

  map-equiv-Σ :
    (e : A ≃ B) (g : (x : A) → C x ≃ D (map-equiv e x)) → Σ A C → Σ B D
  map-equiv-Σ e g = map-equiv (equiv-Σ e g)

Any commuting triangle induces a map on fibers

module _
  {l1 l2 l3 : Level} {X : UU l1} {A : UU l2} {B : UU l3}
  (f : A → X) (g : B → X) (h : A → B) (H : f ~ (g ∘ h))
  where

  fiber-triangle :
    (x : X) → (fiber f x) → (fiber g x)
  pr1 (fiber-triangle .(f a) (pair a refl)) = h a
  pr2 (fiber-triangle .(f a) (pair a refl)) = inv (H a)

  square-tot-fiber-triangle :
    ( h ∘ (map-equiv-total-fiber f)) ~
    ( map-equiv-total-fiber g ∘ tot fiber-triangle)
  square-tot-fiber-triangle (pair .(f a) (pair a refl)) = refl

In a commuting triangle, the top map is an equivalence if and only if it induces an equivalence on fibers

module _
  {l1 l2 l3 : Level} {X : UU l1} {A : UU l2} {B : UU l3}
  (f : A → X) (g : B → X) (h : A → B) (H : f ~ (g ∘ h))
  where

  abstract
    is-fiberwise-equiv-is-equiv-triangle :
      (E : is-equiv h) → is-fiberwise-equiv (fiber-triangle f g h H)
    is-fiberwise-equiv-is-equiv-triangle E =
      is-fiberwise-equiv-is-equiv-tot
        ( is-equiv-top-is-equiv-bottom-square
          ( map-equiv-total-fiber f)
          ( map-equiv-total-fiber g)
          ( tot (fiber-triangle f g h H))
          ( h)
          ( square-tot-fiber-triangle f g h H)
          ( is-equiv-map-equiv-total-fiber f)
          ( is-equiv-map-equiv-total-fiber g)
          ( E))

  abstract
    is-equiv-triangle-is-fiberwise-equiv :
      is-fiberwise-equiv (fiber-triangle f g h H) → is-equiv h
    is-equiv-triangle-is-fiberwise-equiv E =
      is-equiv-bottom-is-equiv-top-square
        ( map-equiv-total-fiber f)
        ( map-equiv-total-fiber g)
        ( tot (fiber-triangle f g h H))
        ( h)
        ( square-tot-fiber-triangle f g h H)
        ( is-equiv-map-equiv-total-fiber f)
        ( is-equiv-map-equiv-total-fiber g)
        ( is-equiv-tot-is-fiberwise-equiv E)

map-Σ preserves identity maps

module _
  {l1 l2 : Level} {A : UU l1} {B : A → UU l2}
  where

  compute-map-Σ-id : map-Σ B id (λ x → id) ~ id
  compute-map-Σ-id = refl-htpy

See also

Recent changes