{-# OPTIONS --universe-polymorphism #-}
module Function.Bijection where
open import Data.Product
open import Level
open import Relation.Binary
open import Function.Equality as F
using (_⟶_) renaming (_∘_ to _⟪∘⟫_)
open import Function.Injection as Inj hiding (id; _∘_)
open import Function.Surjection as Surj hiding (id; _∘_)
record Bijective {f₁ f₂ t₁ t₂}
{From : Setoid f₁ f₂} {To : Setoid t₁ t₂}
(to : From ⟶ To) :
Set (f₁ ⊔ f₂ ⊔ t₁ ⊔ t₂) where
field
injective : Injective to
surjective : Surjective to
open Surjective surjective public
record Bijection {f₁ f₂ t₁ t₂}
(From : Setoid f₁ f₂) (To : Setoid t₁ t₂) :
Set (f₁ ⊔ f₂ ⊔ t₁ ⊔ t₂) where
field
to : From ⟶ To
bijective : Bijective to
open Bijective bijective public
injection : Injection From To
injection = record
{ to = to
; injective = injective
}
surjection : Surjection From To
surjection = record
{ to = to
; surjective = surjective
}
open Surjection surjection public using (right-inverse)
id : ∀ {s₁ s₂} {S : Setoid s₁ s₂} → Bijection S S
id {S = S} = record
{ to = F.id
; bijective = record
{ injective = Injection.injective (Inj.id {S = S})
; surjective = Surjection.surjective (Surj.id {S = S})
}
}
infixr 9 _∘_
_∘_ : ∀ {f₁ f₂ m₁ m₂ t₁ t₂}
{F : Setoid f₁ f₂} {M : Setoid m₁ m₂} {T : Setoid t₁ t₂} →
Bijection M T → Bijection F M → Bijection F T
f ∘ g = record
{ to = to f ⟪∘⟫ to g
; bijective = record
{ injective = Injection.injective (Inj._∘_ (injection f) (injection g))
; surjective = Surjection.surjective (Surj._∘_ (surjection f) (surjection g))
}
} where open Bijection