# Euclid's Lemma for Prime Divisors/Proof 3

## Lemma

Let $p$ be a prime number.

Let $a$ and $b$ be integers such that:

$p \divides a b$

where $\divides$ means is a divisor of.

Then $p \divides a$ or $p \divides b$.

In the words of Euclid:

If two numbers by multiplying one another make some number, and any prime number measure the product, it will also measure one of the original numbers.

## Proof

Let $p \divides a b$.

Suppose $p \nmid a$.

$p \perp a$

As $p \divides a b$, it follows by definition of divisor:

$\exists e \in \Z: e p = a b$
$p : a = b : e$

But as $p \perp a$, by:

Proposition $21$ of Book $\text{VII}$: Coprime Numbers form Fraction in Lowest Terms

and:

Proposition $20$ of Book $\text{VII}$: Ratios of Fractions in Lowest Terms

it follows that:

$p \divides b$

Similarly, if $p \perp b$ then $p \divides a$.

Hence the result.

$\blacksquare$

## Historical Note

This theorem is Proposition $30$ of Book $\text{VII}$ of Euclid's The Elements.