Two Coprime Integers have no Third Integer Proportional
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Theorem
Let $a, b \in \Z_{>0}$ be integers such that $a$ and $b$ are coprime.
Then there is no integer $c \in \Z$ such that:
- $\dfrac a b = \dfrac b c$
In the words of Euclid:
- If two numbers be prime to each other, the second will not be to any other number as the first is to the second.
(The Elements: Book $\text{IX}$: Proposition $16$)
Proof
Suppose such a $c$ exists.
From Coprime Numbers form Fraction in Lowest Terms, $\dfrac a b$ is in canonical form.
From Ratios of Fractions in Lowest Terms:
- $a \divides b$
where $\divides$ denotes divisibility.
This contradicts the fact that $a$ and $b$ are coprime.
Hence such a $c$ cannot exist.
$\blacksquare$
Historical Note
This proof is Proposition $16$ of Book $\text{IX}$ of Euclid's The Elements.
Sources
- 1926: Sir Thomas L. Heath: Euclid: The Thirteen Books of The Elements: Volume 2 (2nd ed.) ... (previous) ... (next): Book $\text{IX}$. Propositions