Smallest Positive Integer Combination is Greatest Common Divisor/Proof 1

Proof
Let $D$ be the subset of $\Z_{>0}$ defined as:
 * $D = \set {a s + b t: s, t \in \Z, a s + b t > 0}$

Setting $s = 1$ and $t = 0$ it is clear that $a = \paren {a \times 1 + b \times 0} \in D$.

So $D \ne \O$.

By Set of Integers Bounded Below by Integer has Smallest Element, $D$ has a smallest element $d$, say.

Thus $d = a s + b t$ for some $s, t \in \Z$.

Proof of $(1)$
From the Division Theorem we can write $a = q d + r$ for some $q, r$ with $0 \le r < d$.

If $r \ne 0$ we have:

So by definition of $D$, it is clear that $r \in D$.

But $r < d$ which contradicts the stipulation that $d$ is the smallest element of $D$.

Thus $r = 0$ and so $a = q d$.

That is $d \divides a$.

By the same argument, it follows that $d \divides b$ also.

Proof of $(2)$
Let $c \divides a$ and $c \divides b$.

Then:
 * $\exists u, v \in \Z: a = c u, b = c v$

Thus:

demonstrating that $c \divides d$.