Injective maps

Content created by Fredrik Bakke, Egbert Rijke and Jonathan Prieto-Cubides.

Created on 2023-02-01.
Last modified on 2023-09-11.

module foundation-core.injective-maps where
Imports 
open import foundation.action-on-identifications-functions
open import foundation.dependent-pair-types
open import foundation.universe-levels

open import foundation-core.contractible-types
open import foundation-core.embeddings
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.propositional-maps
open import foundation-core.propositions
open import foundation-core.sections
open import foundation-core.sets

Idea

A map f : A → B is injective if f x ＝ f y implies x ＝ y.

Warning

The notion of injective map is, however, not homotopically coherent. It is fine to use injectivity for maps between sets, but for maps between general types it is recommended to use the notion of embedding.

Definition

is-injective : {l1 l2 : Level} {A : UU l1} {B : UU l2} → (A → B) → UU (l1 ⊔ l2)
is-injective {l1} {l2} {A} {B} f = {x y : A} → f x ＝ f y → x ＝ y

injection : {l1 l2 : Level} (A : UU l1) (B : UU l2) → UU (l1 ⊔ l2)
injection A B = Σ (A → B) is-injective

Examples

The identity function is injective

is-injective-id : {l1 : Level} {A : UU l1} → is-injective (id {A = A})
is-injective-id p = p

id-injection : {l1 : Level} {A : UU l1} → injection A A
pr1 id-injection = id
pr2 id-injection = is-injective-id

Properties

If a composite is injective, then so is its right factor

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

  is-injective-right-factor :
    (g : B → C) (h : A → B) →
    is-injective (g ∘ h) → is-injective h
  is-injective-right-factor g h is-inj-gh p = is-inj-gh (ap g p)

  is-injective-right-factor-htpy :
    (f : A → C) (g : B → C) (h : A → B) (H : f ~ (g ∘ h)) →
    is-injective f → is-injective h
  is-injective-right-factor-htpy f g h H is-inj-f {x} {x'} p =
    is-inj-f {x} {x'} ((H x) ∙ ((ap g p) ∙ (inv (H x'))))

Injective maps are closed under composition

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

  is-injective-comp :
    {g : B → C} {h : A → B} →
    is-injective h → is-injective g → is-injective (g ∘ h)
  is-injective-comp is-inj-h is-inj-g = is-inj-h ∘ is-inj-g

  is-injective-comp-htpy :
    (f : A → C) (g : B → C) (h : A → B) → f ~ (g ∘ h) →
    is-injective h → is-injective g → is-injective f
  is-injective-comp-htpy f g h H is-inj-h is-inj-g {x} {x'} p =
    is-inj-h (is-inj-g ((inv (H x)) ∙ (p ∙ (H x'))))

Equivalences are injective

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

  abstract
    is-injective-is-equiv : {f : A → B} → is-equiv f → is-injective f
    is-injective-is-equiv H {x} {y} p =
      ( inv (is-retraction-map-inv-is-equiv H x)) ∙
      ( ( ap (map-inv-is-equiv H) p) ∙
        ( is-retraction-map-inv-is-equiv H y))

  abstract
    is-injective-map-equiv : (e : A ≃ B) → is-injective (map-equiv e)
    is-injective-map-equiv (pair f H) = is-injective-is-equiv H

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

  abstract
    is-injective-map-inv-equiv : (e : A ≃ B) → is-injective (map-inv-equiv e)
    is-injective-map-inv-equiv e =
      is-injective-is-equiv (is-equiv-map-inv-equiv e)

  is-equiv-is-injective : {f : A → B} → section f → is-injective f → is-equiv f
  is-equiv-is-injective {f} (pair g G) H =
    is-equiv-is-invertible g G (λ x → H (G (f x)))

Any embedding is injective

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

  is-injective-is-emb : {f : A → B} → is-emb f → is-injective f
  is-injective-is-emb is-emb-f {x} {y} = map-inv-is-equiv (is-emb-f x y)

  is-injective-emb : (e : A ↪ B) → is-injective (map-emb e)
  is-injective-emb e {x} {y} = map-inv-is-equiv (is-emb-map-emb e x y)

Any injective map between sets is an embedding

abstract
  is-emb-is-injective' :
    {l1 l2 : Level} {A : UU l1} (is-set-A : is-set A)
    {B : UU l2} (is-set-B : is-set B) (f : A → B) →
    is-injective f → is-emb f
  is-emb-is-injective' is-set-A is-set-B f is-injective-f x y =
    is-equiv-is-prop
      ( is-set-A x y)
      ( is-set-B (f x) (f y))
      ( is-injective-f)

  is-set-is-injective :
    {l1 l2 : Level} {A : UU l1} {B : UU l2} {f : A → B} →
    is-set B → is-injective f → is-set A
  is-set-is-injective {f = f} H I =
    is-set-prop-in-id
      ( λ x y → f x ＝ f y)
      ( λ x y → H (f x) (f y))
      ( λ x → refl)
      ( λ x y → I)

  is-emb-is-injective :
    {l1 l2 : Level} {A : UU l1} {B : UU l2} {f : A → B} →
    is-set B → is-injective f → is-emb f
  is-emb-is-injective {f = f} H I =
    is-emb-is-injective' (is-set-is-injective H I) H f I

  is-prop-map-is-injective :
    {l1 l2 : Level} {A : UU l1} {B : UU l2} {f : A → B} →
    is-set B → is-injective f → is-prop-map f
  is-prop-map-is-injective {f = f} H I =
    is-prop-map-is-emb (is-emb-is-injective H I)

For a map between sets, being injective is a property

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

  is-prop-is-injective :
    is-set A → (f : A → B) → is-prop (is-injective f)
  is-prop-is-injective H f =
    is-prop-Π'
      ( λ x →
        is-prop-Π'
          ( λ y → is-prop-function-type (H x y)))

  is-injective-Prop : is-set A → (A → B) → Prop (l1 ⊔ l2)
  pr1 (is-injective-Prop H f) = is-injective f
  pr2 (is-injective-Prop H f) = is-prop-is-injective H f

Any map out of a contractible type is injective

is-injective-is-contr :
  {l1 l2 : Level} {A : UU l1} {B : UU l2} (f : A → B) →
  is-contr A → is-injective f
is-injective-is-contr f H p = eq-is-contr H

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