Finite function types

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

Created on 2022-03-31.
Last modified on 2024-02-06.

module univalent-combinatorics.function-types where
Imports
open import foundation.equivalences
open import foundation.universe-levels

open import univalent-combinatorics.cartesian-product-types
open import univalent-combinatorics.counting
open import univalent-combinatorics.dependent-function-types
open import univalent-combinatorics.dependent-pair-types
open import univalent-combinatorics.equality-finite-types
open import univalent-combinatorics.finite-types

Properties

The type of functions between types equipped with a counting can be equipped with a counting

count-function-type :
  {l1 l2 : Level} {A : UU l1} {B : UU l2} 
  count A  count B  count (A  B)
count-function-type e f =
  count-Π e  x  f)

The type of functions between finite types is finite

abstract
  is-finite-function-type :
    {l1 l2 : Level} {A : UU l1} {B : UU l2} 
    is-finite A  is-finite B  is-finite (A  B)
  is-finite-function-type f g = is-finite-Π f  x  g)

_→-𝔽_ : {l1 l2 : Level}  𝔽 l1  𝔽 l2  𝔽 (l1  l2)
pr1 (A →-𝔽 B) = type-𝔽 A  type-𝔽 B
pr2 (A →-𝔽 B) =
  is-finite-function-type (is-finite-type-𝔽 A) (is-finite-type-𝔽 B)

The type of equivalences between finite types is finite

abstract
  is-finite-≃ :
    {l1 l2 : Level} {A : UU l1} {B : UU l2} 
    is-finite A  is-finite B  is-finite (A  B)
  is-finite-≃ f g =
    is-finite-Σ
      ( is-finite-function-type f g)
      ( λ h 
        is-finite-product
          ( is-finite-Σ
            ( is-finite-function-type g f)
            ( λ k 
              is-finite-Π g
                ( λ y  is-finite-eq (has-decidable-equality-is-finite g))))
          ( is-finite-Σ
            ( is-finite-function-type g f)
            ( λ k 
              is-finite-Π f
                ( λ x  is-finite-eq (has-decidable-equality-is-finite f)))))

infix 6 _≃-𝔽_
_≃-𝔽_ : {l1 l2 : Level}  𝔽 l1  𝔽 l2  𝔽 (l1  l2)
pr1 (A ≃-𝔽 B) = type-𝔽 A  type-𝔽 B
pr2 (A ≃-𝔽 B) = is-finite-≃ (is-finite-type-𝔽 A) (is-finite-type-𝔽 B)

The type of automorphisms on a finite type is finite

Aut-𝔽 : {l : Level}  𝔽 l  𝔽 l
Aut-𝔽 A = A ≃-𝔽 A

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