GCD of Integer and Divisor

Theorem
Let $a, b \in \Z_{>0}$, i.e. integers such that $a, b > 0$.

Then:
 * $a \divides b \implies \gcd \set {a, b} = a$

Proof
$a \divides b$ by hypothesis, $a \divides a$ from Integer Divides Itself.

Thus $a$ is a common divisor of $a$ and $b$.

Note that from Integer Absolute Value Greater than Divisors:


 * $\forall x \in \Z: x \divides a \implies x \le \size a$.

As $a$ and $b$ are both positive, the result follows.