Suprema of families of real numbers
Content created by Louis Wasserman.
Created on 2025-08-30.
Last modified on 2025-09-06.
{-# OPTIONS --lossy-unification #-} module real-numbers.suprema-families-real-numbers where
Imports
open import elementary-number-theory.positive-rational-numbers open import foundation.action-on-identifications-functions open import foundation.conjunction open import foundation.coproduct-types open import foundation.dependent-pair-types open import foundation.disjunction open import foundation.empty-types open import foundation.existential-quantification open import foundation.identity-types open import foundation.logical-equivalences open import foundation.propositional-truncations open import foundation.propositions open import foundation.subtypes open import foundation.transport-along-identifications open import foundation.unit-type open import foundation.universe-levels open import order-theory.least-upper-bounds-large-posets open import order-theory.upper-bounds-large-posets open import real-numbers.addition-real-numbers open import real-numbers.binary-maximum-real-numbers open import real-numbers.dedekind-real-numbers open import real-numbers.difference-real-numbers open import real-numbers.inequality-real-numbers open import real-numbers.negation-real-numbers open import real-numbers.positive-real-numbers open import real-numbers.rational-real-numbers open import real-numbers.similarity-real-numbers open import real-numbers.strict-inequality-real-numbers open import real-numbers.subsets-real-numbers
Idea
A real number x is the
supremum¶
of a family y of real numbers indexed by I if x is an
upper bound of y in the
large poset of real numbers and x
is approximated below by the yᵢ: for all ε : ℚ⁺, there
exists an i : I such that
x - yᵢ < ε.
Classically, suprema and least upper bounds are equivalent, but constructively being a supremum is a stronger condition, often required for constructive analysis.
Definition
Suprema of families of real numbers
module _ {l1 l2 : Level} {I : UU l1} (y : I → ℝ l2) where is-approximated-below-prop-family-ℝ : {l3 : Level} → ℝ l3 → Prop (l1 ⊔ l2 ⊔ l3) is-approximated-below-prop-family-ℝ x = Π-Prop ( ℚ⁺) ( λ ε → ∃ I (λ i → le-prop-ℝ (x -ℝ real-ℚ⁺ ε) (y i))) is-approximated-below-family-ℝ : {l3 : Level} → ℝ l3 → UU (l1 ⊔ l2 ⊔ l3) is-approximated-below-family-ℝ x = type-Prop (is-approximated-below-prop-family-ℝ x) is-supremum-prop-family-ℝ : {l3 : Level} → ℝ l3 → Prop (l1 ⊔ l2 ⊔ l3) is-supremum-prop-family-ℝ x = is-upper-bound-prop-family-of-elements-Large-Poset ( ℝ-Large-Poset) ( y) ( x) ∧ is-approximated-below-prop-family-ℝ x is-supremum-family-ℝ : {l3 : Level} → ℝ l3 → UU (l1 ⊔ l2 ⊔ l3) is-supremum-family-ℝ x = type-Prop (is-supremum-prop-family-ℝ x) is-upper-bound-is-supremum-family-ℝ : {l3 : Level} → (x : ℝ l3) → is-supremum-family-ℝ x → is-upper-bound-family-of-elements-Large-Poset ℝ-Large-Poset y x is-upper-bound-is-supremum-family-ℝ _ = pr1 is-approximated-below-is-supremum-family-ℝ : {l3 : Level} → (x : ℝ l3) → is-supremum-family-ℝ x → is-approximated-below-family-ℝ x is-approximated-below-is-supremum-family-ℝ _ = pr2
Suprema of subsets of real numbers
module _ {l1 l2 : Level} (S : subset-ℝ l1 l2) where is-supremum-prop-subset-ℝ : {l3 : Level} → ℝ l3 → Prop (l1 ⊔ lsuc l2 ⊔ l3) is-supremum-prop-subset-ℝ = is-supremum-prop-family-ℝ (inclusion-subset-ℝ S) is-supremum-subset-ℝ : {l3 : Level} → ℝ l3 → UU (l1 ⊔ lsuc l2 ⊔ l3) is-supremum-subset-ℝ x = type-Prop (is-supremum-prop-subset-ℝ x)
Properties
A supremum is a least upper bound
module _ {l1 l2 l3 : Level} {I : UU l1} (y : I → ℝ l2) (x : ℝ l3) (is-supremum-x-yᵢ : is-supremum-family-ℝ y x) where abstract is-least-upper-bound-is-supremum-family-ℝ : is-least-upper-bound-family-of-elements-Large-Poset ( ℝ-Large-Poset) ( y) ( x) pr1 (is-least-upper-bound-is-supremum-family-ℝ z) yᵢ≤z = leq-not-le-ℝ z x ( λ z<x → let open do-syntax-trunc-Prop empty-Prop in do (ε⁺@(ε , _) , ε<x-z) ← exists-ℚ⁺-in-lower-cut-ℝ⁺ (positive-diff-le-ℝ z x z<x) (i , x-ε<yᵢ) ← is-approximated-below-is-supremum-family-ℝ y x is-supremum-x-yᵢ ε⁺ not-leq-le-ℝ z (y i) ( transitive-le-ℝ z (x -ℝ real-ℚ⁺ ε⁺) (y i) ( x-ε<yᵢ) ( le-transpose-left-add-ℝ' _ _ _ ( le-transpose-right-diff-ℝ _ _ _ ( le-real-is-in-lower-cut-ℚ ε (x -ℝ z) ε<x-z)))) ( yᵢ≤z i)) pr2 (is-least-upper-bound-is-supremum-family-ℝ z) x≤z i = transitive-leq-ℝ (y i) x z x≤z ( is-upper-bound-is-supremum-family-ℝ y x is-supremum-x-yᵢ i)
Suprema are unique up to similarity
module _ {l1 l2 : Level} {I : UU l1} (x : I → ℝ l2) where abstract sim-is-supremum-family-ℝ : {l3 : Level} (y : ℝ l3) → is-supremum-family-ℝ x y → {l4 : Level} (z : ℝ l4) → is-supremum-family-ℝ x z → sim-ℝ y z sim-is-supremum-family-ℝ y is-sup-y z is-sup-z = sim-sim-leq-ℝ ( sim-is-least-upper-bound-family-of-elements-Large-Poset ℝ-Large-Poset ( is-least-upper-bound-is-supremum-family-ℝ x y is-sup-y) ( is-least-upper-bound-is-supremum-family-ℝ x z is-sup-z))
Having a supremum at a given universe level is a proposition
module _ {l1 l2 : Level} {I : UU l1} (x : I → ℝ l2) (l3 : Level) where has-supremum-family-ℝ : UU (l1 ⊔ l2 ⊔ lsuc l3) has-supremum-family-ℝ = Σ (ℝ l3) (is-supremum-family-ℝ x) abstract is-prop-has-supremum-family-ℝ : is-prop has-supremum-family-ℝ is-prop-has-supremum-family-ℝ = is-prop-all-elements-equal ( λ (y , is-sup-y) (z , is-sup-z) → eq-type-subtype ( is-supremum-prop-family-ℝ x) ( eq-sim-ℝ (sim-is-supremum-family-ℝ x y is-sup-y z is-sup-z))) has-supremum-prop-family-ℝ : Prop (l1 ⊔ l2 ⊔ lsuc l3) has-supremum-prop-family-ℝ = ( has-supremum-family-ℝ , is-prop-has-supremum-family-ℝ) module _ {l1 l2 : Level} (S : subset-ℝ l1 l2) (l3 : Level) where has-supremum-prop-subset-ℝ : Prop (l1 ⊔ lsuc l2 ⊔ lsuc l3) has-supremum-prop-subset-ℝ = has-supremum-prop-family-ℝ (inclusion-subset-ℝ S) l3 has-supremum-subset-ℝ : UU (l1 ⊔ lsuc l2 ⊔ lsuc l3) has-supremum-subset-ℝ = type-Prop has-supremum-prop-subset-ℝ
A real number r is less than the supremum of the yᵢ if and only if it is less than some yᵢ
module _ {l1 l2 l3 : Level} {I : UU l1} (y : I → ℝ l2) (x : ℝ l3) (is-supremum-x-yᵢ : is-supremum-family-ℝ y x) where abstract le-supremum-le-element-family-ℝ : {l4 : Level} → (z : ℝ l4) (i : I) → le-ℝ z (y i) → le-ℝ z x le-supremum-le-element-family-ℝ z i z<yᵢ = concatenate-le-leq-ℝ z (y i) x z<yᵢ (pr1 is-supremum-x-yᵢ i) le-element-le-supremum-family-ℝ : {l4 : Level} → (z : ℝ l4) → le-ℝ z x → exists I (λ i → le-prop-ℝ z (y i)) le-element-le-supremum-family-ℝ z z<x = let open do-syntax-trunc-Prop (∃ I (λ i → le-prop-ℝ z (y i))) in do (ε⁺@(ε , _) , ε<x-z) ← exists-ℚ⁺-in-lower-cut-ℝ⁺ (positive-diff-le-ℝ z x z<x) (i , x-ε<yᵢ) ← is-approximated-below-is-supremum-family-ℝ y x is-supremum-x-yᵢ ε⁺ intro-exists ( i) ( transitive-le-ℝ z (x -ℝ real-ℚ ε) (y i) ( x-ε<yᵢ) ( le-transpose-left-add-ℝ' _ _ _ ( le-transpose-right-diff-ℝ _ _ _ ( le-real-is-in-lower-cut-ℚ ε (x -ℝ z) ε<x-z)))) le-supremum-iff-le-element-family-ℝ : {l4 : Level} → (z : ℝ l4) → (le-ℝ z x) ↔ (exists I (λ i → le-prop-ℝ z (y i))) pr1 (le-supremum-iff-le-element-family-ℝ z) = le-element-le-supremum-family-ℝ z pr2 (le-supremum-iff-le-element-family-ℝ z) = elim-exists (le-prop-ℝ z x) (le-supremum-le-element-family-ℝ z)
If the families of real numbers f and g have suprema, so does rec-coproduct f g
module _ {l1 l2 l3 l4 l5 : Level} {I : UU l1} {J : UU l2} (f : I → ℝ l3) (g : J → ℝ l3) where sup-rec-coproduct-max-supremum-family-ℝ : (has-supremum-family-ℝ f l4) → (has-supremum-family-ℝ g l5) → ℝ (l4 ⊔ l5) sup-rec-coproduct-max-supremum-family-ℝ (x , _) (y , _) = max-ℝ x y abstract is-upper-bound-rec-coproduct-max-supremum-family-ℝ : (H : has-supremum-family-ℝ f l4) → (K : has-supremum-family-ℝ g l5) → is-upper-bound-family-of-elements-Large-Poset ( ℝ-Large-Poset) ( rec-coproduct f g) ( sup-rec-coproduct-max-supremum-family-ℝ H K) is-upper-bound-rec-coproduct-max-supremum-family-ℝ (x , is-sup-f-x) (y , is-sup-g-y) (inl i) = transitive-leq-ℝ _ _ _ ( leq-left-max-ℝ _ _) ( is-upper-bound-is-supremum-family-ℝ f x is-sup-f-x i) is-upper-bound-rec-coproduct-max-supremum-family-ℝ (x , is-sup-f-x) (y , is-sup-g-y) (inr j) = transitive-leq-ℝ _ _ _ ( leq-right-max-ℝ _ _) ( is-upper-bound-is-supremum-family-ℝ g y is-sup-g-y j) is-approximated-below-rec-coproduct-max-supremum-family-ℝ : (H : has-supremum-family-ℝ f l4) → (K : has-supremum-family-ℝ g l5) → is-approximated-below-family-ℝ ( rec-coproduct f g) ( sup-rec-coproduct-max-supremum-family-ℝ H K) is-approximated-below-rec-coproduct-max-supremum-family-ℝ (x , is-sup-f-x) (y , is-sup-g-y) ε = let (ε₁ , ε₂ , ε₁+ε₂=ε) = split-ℚ⁺ ε open do-syntax-trunc-Prop (∃ (I + J) _) case : {l : Level} → (k : I + J) → (w : ℝ l) → le-ℝ (w -ℝ real-ℚ⁺ ε₂) (rec-coproduct f g k) → le-ℝ (max-ℝ x y -ℝ real-ℚ⁺ ε₁) w → le-ℝ (max-ℝ x y -ℝ real-ℚ⁺ ε) (rec-coproduct f g k) case k w w-ε₂<fgk max-ε₁<w = tr ( λ z → le-ℝ z (rec-coproduct f g k)) ( equational-reasoning (max-ℝ x y -ℝ real-ℚ⁺ ε₁) -ℝ real-ℚ⁺ ε₂ ＝ max-ℝ x y +ℝ (neg-ℝ (real-ℚ⁺ ε₁) -ℝ real-ℚ⁺ ε₂) by associative-add-ℝ _ _ _ ＝ max-ℝ x y -ℝ (real-ℚ⁺ ε₁ +ℝ real-ℚ⁺ ε₂) by ap-add-ℝ refl (inv (distributive-neg-add-ℝ _ _)) ＝ max-ℝ x y -ℝ real-ℚ⁺ (ε₁ +ℚ⁺ ε₂) by ap-diff-ℝ refl (add-real-ℚ _ _) ＝ max-ℝ x y -ℝ real-ℚ⁺ ε by ap-diff-ℝ refl (ap real-ℚ⁺ ε₁+ε₂=ε)) ( transitive-le-ℝ ( (max-ℝ x y -ℝ real-ℚ⁺ ε₁) -ℝ real-ℚ⁺ ε₂) ( w -ℝ real-ℚ⁺ ε₂) ( rec-coproduct f g k) ( w-ε₂<fgk) ( preserves-le-right-add-ℝ _ _ _ max-ε₁<w)) in elim-disjunction ( ∃ _ _) ( λ max-ε₁<x → do (i , x-ε₂<fi) ← is-approximated-below-is-supremum-family-ℝ f x is-sup-f-x ε₂ intro-exists (inl i) (case (inl i) x x-ε₂<fi max-ε₁<x)) ( λ max-ε₁<y → do (j , y-ε₂<gj) ← is-approximated-below-is-supremum-family-ℝ g y is-sup-g-y ε₂ intro-exists (inr j) (case (inr j) y y-ε₂<gj max-ε₁<y)) ( approximate-below-max-ℝ x y ε₁) has-supremum-rec-coproduct-family-ℝ : has-supremum-family-ℝ f l4 → has-supremum-family-ℝ g l5 → has-supremum-family-ℝ (rec-coproduct f g) (l4 ⊔ l5) has-supremum-rec-coproduct-family-ℝ H K = ( sup-rec-coproduct-max-supremum-family-ℝ H K , is-upper-bound-rec-coproduct-max-supremum-family-ℝ H K , is-approximated-below-rec-coproduct-max-supremum-family-ℝ H K)
The supremum of a single real number is the real number
module _ {l : Level} (x : ℝ l) where abstract is-supremum-unit-family-ℝ : is-supremum-family-ℝ {I = unit} (λ _ → x) x is-supremum-unit-family-ℝ = ( ( λ _ → refl-leq-ℝ x) , ( λ ε → intro-exists star (le-diff-real-ℝ⁺ x (positive-real-ℚ⁺ ε))))
See also
External links
Recent changes
- 2025-09-06. Louis Wasserman. Supremum of a coproduct of families of real numbers (#1535).
- 2025-09-03. Louis Wasserman. Maximum and minimum of finitely enumerable subsets of the real numbers (#1523).
- 2025-09-02. Louis Wasserman. Opacify real numbers (#1521).
- 2025-08-30. Louis Wasserman. Suprema and infima of families of real numbers (#1504).