Euclid's Lemma for Irreducible Elements

Lemma
Let $\left({D, +, \times}\right)$ be a Euclidean domain whose unity is $1$.

Let $p$ be an irreducible element of $D$.

Let $a, b \in D$ such that:
 * $p \mathop \backslash a \times b$

where $\backslash$ means is a divisor of.

Then $p \mathop \backslash a$ or $p \mathop \backslash b$.

Proof
Let $p \mathop \backslash a \times b$.

Suppose $p \nmid a$. Then from the definition of irreducible, $p \perp a$.

Thus from Euclid's Lemma for Euclidean Domains it follows that $p \mathop \backslash b$.

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

So:
 * $p \mathop \backslash a b \implies p \mathop \backslash a$ or $p \mathop \backslash b$

as we needed to show.

Also see

 * Euclid's Lemma for Prime Divisors, for the usual statement of this result, which is this lemma as applied specifically to the integers.