The metric space of real numbers

Content created by Egbert Rijke, Fredrik Bakke and malarbol.

Created on 2024-09-28.
Last modified on 2024-10-29.

{-# OPTIONS --lossy-unification #-}

module real-numbers.metric-space-of-real-numbers where
Imports 
open import elementary-number-theory.addition-rational-numbers
open import elementary-number-theory.difference-rational-numbers
open import elementary-number-theory.inequality-rational-numbers
open import elementary-number-theory.positive-rational-numbers
open import elementary-number-theory.rational-numbers
open import elementary-number-theory.strict-inequality-rational-numbers

open import foundation.action-on-identifications-functions
open import foundation.cartesian-product-types
open import foundation.coproduct-types
open import foundation.dependent-pair-types
open import foundation.diagonal-maps-cartesian-products-of-types
open import foundation.empty-types
open import foundation.existential-quantification
open import foundation.function-types
open import foundation.functoriality-cartesian-product-types
open import foundation.identity-types
open import foundation.logical-equivalences
open import foundation.propositions
open import foundation.subtypes
open import foundation.transport-along-identifications
open import foundation.universe-levels

open import metric-spaces.extensional-premetric-structures
open import metric-spaces.isometries-metric-spaces
open import metric-spaces.metric-space-of-rational-numbers
open import metric-spaces.metric-spaces
open import metric-spaces.metric-structures
open import metric-spaces.monotonic-premetric-structures
open import metric-spaces.premetric-spaces
open import metric-spaces.premetric-structures
open import metric-spaces.pseudometric-structures
open import metric-spaces.reflexive-premetric-structures
open import metric-spaces.saturated-metric-spaces
open import metric-spaces.symmetric-premetric-structures
open import metric-spaces.triangular-premetric-structures

open import real-numbers.dedekind-real-numbers
open import real-numbers.rational-real-numbers

Idea

The type of real numbers equipped with the premetric where x y : ℝ are d-neighbors when for any r : ℚ the following two conditions hold:

  • if r + d is in the lower cut of y, r is in the lower cut of x
  • if r + d is in the lower cut of x, r is in the lower cut of y

is a saturated metric space. It is the standard metric space of real numbers.

Definitions

The standard premetric on the real numbers

module _
  {l : Level} (d : ℚ⁺) (x y : ℝ l)
  where

  is-in-lower-neighborhood-leq-prop-ℝ : Prop l
  is-in-lower-neighborhood-leq-prop-ℝ =
    Π-Prop
      ( ℚ)
      ( λ r →
        hom-Prop
          ( lower-cut-ℝ y (r +ℚ (rational-ℚ⁺ d)))
            ( lower-cut-ℝ x r))

  is-in-lower-neighborhood-leq-ℝ : UU l
  is-in-lower-neighborhood-leq-ℝ =
    type-Prop is-in-lower-neighborhood-leq-prop-ℝ

premetric-leq-ℝ : (l : Level) → Premetric l (ℝ l)
premetric-leq-ℝ l d x y =
  product-Prop
    ( is-in-lower-neighborhood-leq-prop-ℝ d x y)
    ( is-in-lower-neighborhood-leq-prop-ℝ d y x)

Properties

The standard premetric on the real numbers is a metric structure

The triangle inequality is the 91st theorem on Freek Wiedijk’s list of 100 theorems [Wie].

is-reflexive-premetric-leq-ℝ :
  {l : Level} → is-reflexive-Premetric (premetric-leq-ℝ l)
is-reflexive-premetric-leq-ℝ d x =
  diagonal-product
    ( (r : ℚ) →
      is-in-lower-cut-ℝ x (r +ℚ (rational-ℚ⁺ d)) → is-in-lower-cut-ℝ x r)
    ( λ r →
      le-lower-cut-ℝ x r (r +ℚ rational-ℚ⁺ d) (le-right-add-rational-ℚ⁺ r d))

is-symmetric-premetric-leq-ℝ :
  {l : Level} → is-symmetric-Premetric (premetric-leq-ℝ l)
is-symmetric-premetric-leq-ℝ d x y (H , K) = (K , H)

is-tight-premetric-leq-ℝ : {l : Level} → is-tight-Premetric (premetric-leq-ℝ l)
is-tight-premetric-leq-ℝ x y H =
  eq-eq-lower-cut-ℝ
    ( x)
    ( y)
    ( antisymmetric-leq-subtype
      ( lower-cut-ℝ x)
      ( lower-cut-ℝ y)
      ( λ r Lxr →
        elim-exists
          ( lower-cut-ℝ y r)
          ( λ s (r<s , Lxs) →
            pr2
              ( H (positive-diff-le-ℚ r s r<s))
              ( r)
              ( inv-tr
                ( λ u → is-in-lower-cut-ℝ x u)
                ( right-law-positive-diff-le-ℚ r s r<s)
                ( Lxs)))
          ( forward-implication (is-rounded-lower-cut-ℝ x r) Lxr))
      ( λ r Lyr →
        elim-exists
          ( lower-cut-ℝ x r)
          ( λ s (r<s , Lys) →
            pr1
              ( H (positive-diff-le-ℚ r s r<s))
              ( r)
              ( inv-tr
                ( λ u → is-in-lower-cut-ℝ y u)
                ( right-law-positive-diff-le-ℚ r s r<s)
                ( Lys)))
          ( forward-implication (is-rounded-lower-cut-ℝ y r) Lyr)))

is-local-premetric-leq-ℝ : {l : Level} → is-local-Premetric (premetric-leq-ℝ l)
is-local-premetric-leq-ℝ {l} =
  is-local-is-tight-Premetric
    ( premetric-leq-ℝ l)
    ( is-tight-premetric-leq-ℝ)

is-triangular-premetric-leq-ℝ :
  {l : Level} → is-triangular-Premetric (premetric-leq-ℝ l)
pr1 (is-triangular-premetric-leq-ℝ x y z dxy dyz Hyz Hxy) r =
  pr1 Hxy r ∘
  pr1 Hyz (r +ℚ rational-ℚ⁺ dxy) ∘
  inv-tr
    ( is-in-lower-cut-ℝ z)
    ( associative-add-ℚ r (rational-ℚ⁺ dxy) (rational-ℚ⁺ dyz))
pr2 (is-triangular-premetric-leq-ℝ x y z dxy dyz Hyz Hxy) r =
  pr2 Hyz r ∘
  pr2 Hxy (r +ℚ rational-ℚ⁺ dyz) ∘
  inv-tr
    ( is-in-lower-cut-ℝ x)
    ( associative-add-ℚ r (rational-ℚ⁺ dyz) (rational-ℚ⁺ dxy) ∙
      ap (add-ℚ r) (commutative-add-ℚ (rational-ℚ⁺ dyz) (rational-ℚ⁺ dxy)))

is-pseudometric-premetric-leq-ℝ :
  {l : Level} → is-pseudometric-Premetric (premetric-leq-ℝ l)
is-pseudometric-premetric-leq-ℝ =
  is-reflexive-premetric-leq-ℝ ,
  is-symmetric-premetric-leq-ℝ ,
  is-triangular-premetric-leq-ℝ

is-metric-premetric-leq-ℝ :
  {l : Level} → is-metric-Premetric (premetric-leq-ℝ l)
pr1 is-metric-premetric-leq-ℝ = is-pseudometric-premetric-leq-ℝ
pr2 is-metric-premetric-leq-ℝ = is-local-premetric-leq-ℝ

The standard saturated metric space of real numbers

module _
  (l : Level)
  where

  premetric-space-leq-ℝ : Premetric-Space (lsuc l) l
  premetric-space-leq-ℝ = ℝ l , premetric-leq-ℝ l

  metric-space-leq-ℝ : Metric-Space (lsuc l) l
  pr1 metric-space-leq-ℝ = premetric-space-leq-ℝ
  pr2 metric-space-leq-ℝ = is-metric-premetric-leq-ℝ

Properties

The standard metric space of real numbers is saturated

module _
  {l : Level} (x y : ℝ l) (ε : ℚ⁺)
  (H : (δ : ℚ⁺) → is-in-lower-neighborhood-leq-ℝ (ε +ℚ⁺ δ) x y)
  where

  is-closed-lower-neighborhood-leq-ℝ :
    is-in-lower-neighborhood-leq-ℝ ε x y
  is-closed-lower-neighborhood-leq-ℝ r I =
    elim-exists
      ( lower-cut-ℝ x r)
      ( λ r' (K , I') →
        H ( positive-diff-le-ℚ (r +ℚ rational-ℚ⁺ ε) r' K)
          ( r)
          ( tr
            ( is-in-lower-cut-ℝ y)
            ( ( inv
                ( right-law-positive-diff-le-ℚ
                  ( r +ℚ rational-ℚ⁺ ε)
                  ( r')
                  ( K))) ∙
              ( associative-add-ℚ
                ( r)
                ( rational-ℚ⁺ ε)
                    ( r' -ℚ (r +ℚ rational-ℚ⁺ ε))))
            ( I')))
      ( forward-implication
        ( is-rounded-lower-cut-ℝ y (r +ℚ rational-ℚ⁺ ε)) I)

module _
  {l : Level}
  where

  is-saturated-metric-space-leq-ℝ :
    is-saturated-Metric-Space (metric-space-leq-ℝ l)
  is-saturated-metric-space-leq-ℝ ε x y H =
    ( is-closed-lower-neighborhood-leq-ℝ x y ε (pr1 ∘ H)) ,
    ( is-closed-lower-neighborhood-leq-ℝ y x ε (pr2 ∘ H))

Tha canonical embedding from rational to real numbers is an isometry between metric spaces

is-isometry-metric-space-leq-real-ℚ :
  is-isometry-Metric-Space
    ( metric-space-leq-ℚ)
    ( metric-space-leq-ℝ lzero)
    ( real-ℚ)
is-isometry-metric-space-leq-real-ℚ d x y =
  pair
    ( map-product
      ( le-le-add-positive-leq-add-positive-ℚ x y d)
      ( le-le-add-positive-leq-add-positive-ℚ y x d))
    ( map-product
      ( leq-add-positive-le-le-add-positive-ℚ x y d)
      ( leq-add-positive-le-le-add-positive-ℚ y x d))

References

[Wie]
Freek Wiedijk. Formalizing 100 theorems. URL: https://www.cs.ru.nl/~freek/100/.

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