# Euclid's Lemma for Prime Divisors

## 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 $p \divides b$.

### General Result

Let $p$ be a prime number.

Let $\ds n = \prod_{i \mathop = 1}^r a_i$.

Then if $p$ divides $n$, it follows that $p$ divides $a_i$ for some $i$ such that $1 \le i \le r$.

That is:

$p \divides a_1 a_2 \ldots a_n \implies p \divides a_1 \lor p \divides a_2 \lor \cdots \lor p \divides a_n$

### Corollary

Let $p, p_1, p_2, \ldots, p_n$ be primes such that:

$p \divides \ds \prod_{i \mathop = 1}^n p_i$

Then:

$\exists i \in \closedint 1 n: p = p_i$

## Proof 1

We have that the integers form a Euclidean domain.

Then from Irreducible Elements of Ring of Integers we have that the irreducible elements of $\Z$ are the primes and their negatives.

The result then follows directly from Euclid's Lemma for Irreducible Elements.

$\blacksquare$

## Proof 2

Let $p \divides a b$.

Suppose $p \nmid a$.

Then from the definition of prime:

$p \perp a$

where $\perp$ indicates that $p$ and $a$ are coprime.

Thus from Euclid's Lemma it follows that:

$p \divides b$

Similarly, if $p \nmid b$ it follows that $p \divides a$.

So:

$p \divides a b \implies p \divides a$ or $p \divides b$

as we needed to show.

$\blacksquare$

## Proof 3

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.

$\blacksquare$

## Also presented as

Some sources present this as:

Let $p$ be a prime number.

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

$a b \equiv 0 \pmod p$

Then either $a \equiv 0 \pmod p$ or $b \equiv 0 \pmod p$.

## Also known as

Some sources give the name of this as Euclid's first theorem.

## Source of Name

This entry was named for Euclid.

## Also see

Some sources use this property to define a prime number.