# Injective maps

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

Created on 2023-02-01.

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