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...lementary-number-theory/archimedean-property-positive-rational-numbers.lagda.md
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# The Archimedean property of the positive rational numbers | ||
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```agda | ||
{-# OPTIONS --lossy-unification #-} | ||
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module elementary-number-theory.archimedean-property-positive-rational-numbers where | ||
``` | ||
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<details><summary>Imports</summary> | ||
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```agda | ||
open import elementary-number-theory.archimedean-property-rational-numbers | ||
open import elementary-number-theory.integers | ||
open import elementary-number-theory.multiplication-rational-numbers | ||
open import elementary-number-theory.multiplicative-group-of-positive-rational-numbers | ||
open import elementary-number-theory.natural-numbers | ||
open import elementary-number-theory.nonzero-natural-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 | ||
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open import foundation.action-on-identifications-functions | ||
open import foundation.binary-transport | ||
open import foundation.dependent-pair-types | ||
open import foundation.existential-quantification | ||
open import foundation.identity-types | ||
open import foundation.propositional-truncations | ||
open import foundation.transport-along-identifications | ||
``` | ||
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</details> | ||
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## Definition | ||
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The | ||
{{#concept "Archimedean property" Disambiguation="positive rational numbers" Agda=archimedean-property-ℚ⁺}} | ||
of `ℚ⁺` is that for any two | ||
[positive rational numbers](elementary-number-theory.positive-rational-numbers.md) | ||
`x y : ℚ⁺`, there is a | ||
[nonzero natural number](elementary-number-theory.nonzero-natural-numbers.md) | ||
`n` such that `y` is | ||
[less than](elementary-number-theory.strict-inequality-rational-numbers.md) `n` | ||
times `x`. | ||
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```agda | ||
abstract | ||
bound-archimedean-property-ℚ⁺ : | ||
(x y : ℚ⁺) → | ||
Σ ℕ⁺ (λ n → le-ℚ⁺ y (positive-rational-ℕ⁺ n *ℚ⁺ x)) | ||
bound-archimedean-property-ℚ⁺ (x , pos-x) (y , pos-y) = | ||
let | ||
(n , y<nx) = bound-archimedean-property-ℚ x y pos-x | ||
n≠0 : is-nonzero-ℕ n | ||
n≠0 n=0 = | ||
asymmetric-le-ℚ | ||
( zero-ℚ) | ||
( y) | ||
( le-zero-is-positive-ℚ y pos-y) | ||
( tr | ||
( le-ℚ y) | ||
( equational-reasoning | ||
rational-ℤ (int-ℕ n) *ℚ x | ||
= rational-ℤ (int-ℕ 0) *ℚ x | ||
by ap (λ m → rational-ℤ (int-ℕ m) *ℚ x) n=0 | ||
= zero-ℚ by left-zero-law-mul-ℚ x) | ||
y<nx) | ||
in (n , n≠0) , y<nx | ||
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archimedean-property-ℚ⁺ : | ||
(x y : ℚ⁺) → | ||
exists ℕ⁺ (λ n → le-prop-ℚ⁺ y (positive-rational-ℕ⁺ n *ℚ⁺ x)) | ||
archimedean-property-ℚ⁺ x y = | ||
unit-trunc-Prop (bound-archimedean-property-ℚ⁺ x y) | ||
``` |
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