Apartness relations

Content created by Fredrik Bakke, Egbert Rijke, Jonathan Prieto-Cubides, Julian KG, fernabnor and louismntnu.

Created on 2022-06-12.
Last modified on 2024-05-23.

module foundation.apartness-relations where
Imports 
open import foundation.binary-relations
open import foundation.dependent-pair-types
open import foundation.disjunction
open import foundation.existential-quantification
open import foundation.propositional-truncations
open import foundation.universal-quantification
open import foundation.universe-levels

open import foundation-core.cartesian-product-types
open import foundation-core.coproduct-types
open import foundation-core.empty-types
open import foundation-core.identity-types
open import foundation-core.negation
open import foundation-core.propositions

Idea

An apartness relation on a type A is a relation R which is

  • Antireflexive: For any a : A we have ¬ (R a a)
  • Symmetric: For any a b : A we have R a b → R b a
  • Cotransitive: For any a b c : A we have R a b → R a c ∨ R b c.

The idea of an apartness relation R is that R a b holds if you can positively establish a difference between a and b. For example, two subsets A and B of a type X are apart if we can find an element that is in the symmetric difference of A and B.

Definitions

Apartness relations

module _
  {l1 l2 : Level} {A : UU l1} (R : A → A → Prop l2)
  where

  is-antireflexive : UU (l1 ⊔ l2)
  is-antireflexive = (a : A) → ¬ (type-Prop (R a a))

  is-consistent : UU (l1 ⊔ l2)
  is-consistent = (a b : A) → (a ＝ b) → ¬ (type-Prop (R a b))

  is-cotransitive-Prop : Prop (l1 ⊔ l2)
  is-cotransitive-Prop =
    ∀' A (λ a → ∀' A (λ b → ∀' A (λ c → R a b ⇒ (R a c ∨ R b c))))

  is-cotransitive : UU (l1 ⊔ l2)
  is-cotransitive = type-Prop is-cotransitive-Prop

  is-apartness-relation : UU (l1 ⊔ l2)
  is-apartness-relation =
    ( is-antireflexive) ×
    ( is-symmetric (λ x y → type-Prop (R x y))) ×
    ( is-cotransitive)

Apartness-Relation : {l1 : Level} (l2 : Level) (A : UU l1) → UU (l1 ⊔ lsuc l2)
Apartness-Relation l2 A =
  Σ (Relation-Prop l2 A) (is-apartness-relation)

module _
  {l1 l2 : Level} {A : UU l1} (R : Apartness-Relation l2 A)
  where

  rel-Apartness-Relation : A → A → Prop l2
  rel-Apartness-Relation = pr1 R

  apart-Apartness-Relation : A → A → UU l2
  apart-Apartness-Relation x y = type-Prop (rel-Apartness-Relation x y)

  antirefl-Apartness-Relation : is-antireflexive rel-Apartness-Relation
  antirefl-Apartness-Relation = pr1 (pr2 R)

  consistent-Apartness-Relation : is-consistent rel-Apartness-Relation
  consistent-Apartness-Relation x .x refl =
    antirefl-Apartness-Relation x

  symmetric-Apartness-Relation : is-symmetric apart-Apartness-Relation
  symmetric-Apartness-Relation = pr1 (pr2 (pr2 R))

  cotransitive-Apartness-Relation : is-cotransitive rel-Apartness-Relation
  cotransitive-Apartness-Relation = pr2 (pr2 (pr2 R))

Types equipped with apartness relation

Type-With-Apartness :
  (l1 l2 : Level) → UU (lsuc l1 ⊔ lsuc l2)
Type-With-Apartness l1 l2 =
  Σ (UU l1) (Apartness-Relation l2)

module _
  {l1 l2 : Level} (A : Type-With-Apartness l1 l2)
  where

  type-Type-With-Apartness : UU l1
  type-Type-With-Apartness = pr1 A

  apartness-relation-Type-With-Apartness :
    Apartness-Relation l2 type-Type-With-Apartness
  apartness-relation-Type-With-Apartness = pr2 A

  rel-apart-Type-With-Apartness : Relation-Prop l2 type-Type-With-Apartness
  rel-apart-Type-With-Apartness =
    rel-Apartness-Relation apartness-relation-Type-With-Apartness

  apart-Type-With-Apartness :
    (x y : type-Type-With-Apartness) → UU l2
  apart-Type-With-Apartness =
    apart-Apartness-Relation apartness-relation-Type-With-Apartness

  antirefl-apart-Type-With-Apartness :
    is-antireflexive rel-apart-Type-With-Apartness
  antirefl-apart-Type-With-Apartness =
    antirefl-Apartness-Relation apartness-relation-Type-With-Apartness

  consistent-apart-Type-With-Apartness :
    is-consistent rel-apart-Type-With-Apartness
  consistent-apart-Type-With-Apartness =
    consistent-Apartness-Relation apartness-relation-Type-With-Apartness

  symmetric-apart-Type-With-Apartness :
    is-symmetric apart-Type-With-Apartness
  symmetric-apart-Type-With-Apartness =
    symmetric-Apartness-Relation apartness-relation-Type-With-Apartness

  cotransitive-apart-Type-With-Apartness :
    is-cotransitive rel-apart-Type-With-Apartness
  cotransitive-apart-Type-With-Apartness =
    cotransitive-Apartness-Relation apartness-relation-Type-With-Apartness

Apartness on the type of functions into a type with an apartness relation

module _
  {l1 l2 l3 : Level} (X : UU l1) (Y : Type-With-Apartness l2 l3)
  where

  rel-apart-function-into-Type-With-Apartness :
    Relation-Prop (l1 ⊔ l3) (X → type-Type-With-Apartness Y)
  rel-apart-function-into-Type-With-Apartness f g =
    ∃ X (λ x → rel-apart-Type-With-Apartness Y (f x) (g x))

  apart-function-into-Type-With-Apartness :
    Relation (l1 ⊔ l3) (X → type-Type-With-Apartness Y)
  apart-function-into-Type-With-Apartness f g =
    type-Prop (rel-apart-function-into-Type-With-Apartness f g)

  is-prop-apart-function-into-Type-With-Apartness :
    (f g : X → type-Type-With-Apartness Y) →
    is-prop (apart-function-into-Type-With-Apartness f g)
  is-prop-apart-function-into-Type-With-Apartness f g =
    is-prop-type-Prop (rel-apart-function-into-Type-With-Apartness f g)

Properties

module _
  {l1 l2 l3 : Level} (X : UU l1) (Y : Type-With-Apartness l2 l3)
  where

  is-antireflexive-apart-function-into-Type-With-Apartness :
    is-antireflexive (rel-apart-function-into-Type-With-Apartness X Y)
  is-antireflexive-apart-function-into-Type-With-Apartness f H =
    apply-universal-property-trunc-Prop H
      ( empty-Prop)
      ( λ (x , a) → antirefl-apart-Type-With-Apartness Y (f x) a)

  is-symmetric-apart-function-into-Type-With-Apartness :
    is-symmetric (apart-function-into-Type-With-Apartness X Y)
  is-symmetric-apart-function-into-Type-With-Apartness f g H =
    apply-universal-property-trunc-Prop H
      ( rel-apart-function-into-Type-With-Apartness X Y g f)
      ( λ (x , a) →
        unit-trunc-Prop
          ( x , symmetric-apart-Type-With-Apartness Y (f x) (g x) a))

  abstract
    is-cotransitive-apart-function-into-Type-With-Apartness :
      is-cotransitive (rel-apart-function-into-Type-With-Apartness X Y)
    is-cotransitive-apart-function-into-Type-With-Apartness f g h H =
      apply-universal-property-trunc-Prop H
        ( disjunction-Prop
          ( rel-apart-function-into-Type-With-Apartness X Y f h)
          ( rel-apart-function-into-Type-With-Apartness X Y g h))
        ( λ (x , a) →
          apply-universal-property-trunc-Prop
            ( cotransitive-apart-Type-With-Apartness Y (f x) (g x) (h x) a)
            ( disjunction-Prop
              ( rel-apart-function-into-Type-With-Apartness X Y f h)
              ( rel-apart-function-into-Type-With-Apartness X Y g h))
            ( λ where
              ( inl b) → inl-disjunction (intro-exists x b)
              ( inr b) → inr-disjunction (intro-exists x b)))

  exp-Type-With-Apartness : Type-With-Apartness (l1 ⊔ l2) (l1 ⊔ l3)
  pr1 exp-Type-With-Apartness = X → type-Type-With-Apartness Y
  pr1 (pr2 exp-Type-With-Apartness) =
    rel-apart-function-into-Type-With-Apartness X Y
  pr1 (pr2 (pr2 exp-Type-With-Apartness)) =
    is-antireflexive-apart-function-into-Type-With-Apartness
  pr1 (pr2 (pr2 (pr2 exp-Type-With-Apartness))) =
    is-symmetric-apart-function-into-Type-With-Apartness
  pr2 (pr2 (pr2 (pr2 exp-Type-With-Apartness))) =
    is-cotransitive-apart-function-into-Type-With-Apartness

References

[MRR88]
Ray Mines, Fred Richman, and Wim Ruitenburg. A Course in Constructive Algebra. Universitext. Springer New York, 1988. ISBN 978-0-387-96640-3 978-1-4419-8640-5. URL: http://link.springer.com/10.1007/978-1-4419-8640-5, doi:10.1007/978-1-4419-8640-5.

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