Coproducts of finite types

Content created by Egbert Rijke, Fredrik Bakke, Jonathan Prieto-Cubides, Eléonore Mangel, Elisabeth Bonnevier, Maša Žaucer and Victor Blanchi.

Created on 2022-03-14.
Last modified on 2023-06-10.

module univalent-combinatorics.coproduct-types where
open import elementary-number-theory.addition-natural-numbers
open import elementary-number-theory.natural-numbers

open import foundation.action-on-identifications-functions
open import foundation.coproduct-types
open import foundation.dependent-pair-types
open import foundation.equivalences
open import foundation.function-types
open import foundation.functoriality-coproduct-types
open import foundation.functoriality-propositional-truncation
open import foundation.homotopies
open import foundation.identity-types
open import foundation.mere-equivalences
open import foundation.propositional-truncations
open import foundation.type-arithmetic-coproduct-types
open import foundation.type-arithmetic-empty-type
open import foundation.universe-levels

open import univalent-combinatorics.counting
open import univalent-combinatorics.counting-decidable-subtypes
open import univalent-combinatorics.double-counting
open import univalent-combinatorics.finite-types
open import univalent-combinatorics.standard-finite-types


Coproducts of finite types are finite, giving a coproduct operation on the type 𝔽 of finite types.


The standard finite types are closed under coproducts

coprod-Fin :
  (k l : )  (Fin k + Fin l)  Fin (k +ℕ l)
coprod-Fin k zero-ℕ = right-unit-law-coprod (Fin k)
coprod-Fin k (succ-ℕ l) =
  (equiv-coprod (coprod-Fin k l) id-equiv) ∘e inv-associative-coprod

map-coprod-Fin :
  (k l : )  (Fin k + Fin l)  Fin (k +ℕ l)
map-coprod-Fin k l = map-equiv (coprod-Fin k l)

Fin-add-ℕ :
  (k l : )  Fin (k +ℕ l)  (Fin k + Fin l)
Fin-add-ℕ k l = inv-equiv (coprod-Fin k l)

inl-coprod-Fin :
  (k l : )  Fin k  Fin (k +ℕ l)
inl-coprod-Fin k l = map-coprod-Fin k l  inl

inr-coprod-Fin :
  (k l : )  Fin l  Fin (k +ℕ l)
inr-coprod-Fin k l = map-coprod-Fin k l  inr

compute-inl-coprod-Fin :
  (k : )  inl-coprod-Fin k 0 ~ id
compute-inl-coprod-Fin k x = refl

Inclusion of coprod-Fin into the natural numbers

nat-coprod-Fin :
  (n m : )  (x : Fin n + Fin m) 
  nat-Fin (n +ℕ m) (map-coprod-Fin n m x) 
  ind-coprod _ (nat-Fin n)  i  n +ℕ (nat-Fin m i)) x
nat-coprod-Fin n zero-ℕ (inl x) = refl
nat-coprod-Fin n (succ-ℕ m) (inl x) = nat-coprod-Fin n m (inl x)
nat-coprod-Fin n (succ-ℕ m) (inr (inl x)) = nat-coprod-Fin n m (inr x)
nat-coprod-Fin n (succ-ℕ m) (inr (inr star)) = refl

nat-inl-coprod-Fin :
  (n m : ) (i : Fin n) 
  nat-Fin (n +ℕ m) (inl-coprod-Fin n m i)  nat-Fin n i
nat-inl-coprod-Fin n m i = nat-coprod-Fin n m (inl i)

nat-inr-coprod-Fin :
  (n m : ) (i : Fin m) 
  nat-Fin (n +ℕ m) (inr-coprod-Fin n m i)  n +ℕ (nat-Fin m i)
nat-inr-coprod-Fin n m i = nat-coprod-Fin n m (inr i)

Types equipped with a count are closed under coproducts

count-coprod :
  {l1 l2 : Level} {X : UU l1} {Y : UU l2} 
  count X  count Y  count (X + Y)
pr1 (count-coprod (pair k e) (pair l f)) = k +ℕ l
pr2 (count-coprod (pair k e) (pair l f)) =
  (equiv-coprod e f) ∘e (inv-equiv (coprod-Fin k l))

  number-of-elements-count-coprod :
    {l1 l2 : Level} {X : UU l1} {Y : UU l2} (e : count X) (f : count Y) 
    Id ( number-of-elements-count (count-coprod e f))
      ( (number-of-elements-count e) +ℕ (number-of-elements-count f))
  number-of-elements-count-coprod (pair k e) (pair l f) = refl

If both Σ A P and Σ A Q have a count, then Σ A P + Q have a count

count-Σ-coprod :
  {l1 l2 l3 : Level} {A : UU l1} {P : A  UU l2} {Q : A  UU l3} 
  count (Σ A P)  count (Σ A Q)  count (Σ A  x  (P x) + (Q x)))
pr1 (count-Σ-coprod count-P count-Q) = pr1 (count-coprod count-P count-Q)
pr2 (count-Σ-coprod count-P count-Q) =
  ( inv-equiv (left-distributive-Σ-coprod _ _ _)) ∘e
  ( pr2 (count-coprod count-P count-Q))

If X + Y has a count, then both X and Y have a count

module _
  {l1 l2 : Level} {X : UU l1} {Y : UU l2}

  count-left-summand : count (X + Y)  count X
  count-left-summand e =
      ( equiv-left-summand)
      ( count-decidable-subtype is-left-Decidable-Prop e)

  count-right-summand : count (X + Y)  count Y
  count-right-summand e =
      ( equiv-right-summand)
      ( count-decidable-subtype is-right-Decidable-Prop e)

If each of A, B, and A + B come equipped with countings, then the number of elements of A and of B add up to the number of elements of A + B

  double-counting-coprod :
    { l1 l2 : Level} {A : UU l1} {B : UU l2}
    ( count-A : count A) (count-B : count B) (count-C : count (A + B)) 
      ( number-of-elements-count count-C)
      ( number-of-elements-count count-A +ℕ number-of-elements-count count-B)
  double-counting-coprod count-A count-B count-C =
    ( double-counting count-C (count-coprod count-A count-B)) 
    ( number-of-elements-count-coprod count-A count-B)

  sum-number-of-elements-coprod :
    {l1 l2 : Level} {A : UU l1} {B : UU l2} (e : count (A + B)) 
      ( ( number-of-elements-count (count-left-summand e)) +ℕ
        ( number-of-elements-count (count-right-summand e)))
      ( number-of-elements-count e)
  sum-number-of-elements-coprod e =
    ( inv
      ( number-of-elements-count-coprod
        ( count-left-summand e)
        ( count-right-summand e))) 
    ( inv
      ( double-counting-coprod
        ( count-left-summand e)
        ( count-right-summand e) e))

Finite types are closed under coproducts

  is-finite-coprod :
    {l1 l2 : Level} {X : UU l1} {Y : UU l2} 
    is-finite X  is-finite Y  is-finite (X + Y)
  is-finite-coprod {X = X} {Y} is-finite-X is-finite-Y =
    apply-universal-property-trunc-Prop is-finite-X
      ( is-finite-Prop (X + Y))
      ( λ (e : count X) 
        apply-universal-property-trunc-Prop is-finite-Y
          ( is-finite-Prop (X + Y))
          ( is-finite-count  (count-coprod e)))

coprod-𝔽 : {l1 l2 : Level}  𝔽 l1  𝔽 l2  𝔽 (l1  l2)
pr1 (coprod-𝔽 X Y) = (type-𝔽 X) + (type-𝔽 Y)
pr2 (coprod-𝔽 X Y) = is-finite-coprod (is-finite-type-𝔽 X) (is-finite-type-𝔽 Y)

  is-finite-left-summand :
    {l1 l2 : Level} {X : UU l1} {Y : UU l2}  is-finite (X + Y) 
    is-finite X
  is-finite-left-summand =
    map-trunc-Prop count-left-summand

  is-finite-right-summand :
    {l1 l2 : Level} {X : UU l1} {Y : UU l2}  is-finite (X + Y) 
    is-finite Y
  is-finite-right-summand =
    map-trunc-Prop count-right-summand

coprod-UU-Fin :
  {l1 l2 : Level} (k l : )  UU-Fin l1 k  UU-Fin l2 l 
  UU-Fin (l1  l2) (k +ℕ l)
pr1 (coprod-UU-Fin {l1} {l2} k l (pair X H) (pair Y K)) = X + Y
pr2 (coprod-UU-Fin {l1} {l2} k l (pair X H) (pair Y K)) =
  apply-universal-property-trunc-Prop H
    ( mere-equiv-Prop (Fin (k +ℕ l)) (X + Y))
    ( λ e1 
      apply-universal-property-trunc-Prop K
        ( mere-equiv-Prop (Fin (k +ℕ l)) (X + Y))
        ( λ e2 
            ( equiv-coprod e1 e2 ∘e inv-equiv (coprod-Fin k l))))

coprod-eq-is-finite :
  {l1 l2 : Level} {X : UU l1} {Y : UU l2} (P : is-finite X) (Q : is-finite Y) 
      ( (number-of-elements-is-finite P) +ℕ (number-of-elements-is-finite Q))
      ( number-of-elements-is-finite (is-finite-coprod P Q))
coprod-eq-is-finite {X = X} {Y = Y} P Q =
    ( number-of-elements-has-finite-cardinality)
    ( all-elements-equal-has-finite-cardinality
      ( pair
        ( number-of-elements-is-finite P +ℕ number-of-elements-is-finite Q)
        ( has-cardinality-type-UU-Fin
          ( number-of-elements-is-finite P +ℕ number-of-elements-is-finite Q)
          ( coprod-UU-Fin
            ( number-of-elements-is-finite P)
            ( number-of-elements-is-finite Q)
            ( pair X
              ( mere-equiv-has-finite-cardinality
                ( has-finite-cardinality-is-finite P)))
            ( pair Y
              ( mere-equiv-has-finite-cardinality
                ( has-finite-cardinality-is-finite Q))))))
      ( has-finite-cardinality-is-finite (is-finite-coprod P Q)))

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