UFD is GCD Domain

Theorem
Let $A$ be a unique factorisation domain.

Then $A$ is a GCD domain.

Proof
Let $x \mathop \backslash y$ denote $x$ divides $y$.

Let $x,y \in A$, with factorizations:


 * $x = u x_1 \cdots x_r,\quad y = v y_1 \cdots y_s$

with $u,v$ units and the $x_i$, $y_i$ irreducible.

We arrange the factorizations as follows:


 * $x = u \left({x_1 \cdots x_t}\right) x_{t + 1} \cdots x_r$


 * $y = v \left({y_1 \cdots y_t}\right) y_{t + 1} \cdots y_s$

where:
 * $t \le \operatorname{min} \left\{{r, s}\right\}$
 * For $i = 1, \ldots, t$, $x_i$ and $y_i$ are associates
 * For any $i \in \left\{{t+1, \ldots, r}\right\}$, $j \in \left\{{t+1, \ldots, s}\right\}$, $x_i$ and $y_j$ are not associates.

Let $d = x_1 \cdots x_t$ (recall that the empty product is $1$, i.e. $d = 1$ when $t = 0$).

We claim that $d$ is a greatest common divisor for $x$ and $y$.

Certainly $d \mathop \backslash x$ and $d \mathop \backslash y$, so let $f$ be another common divisor of $x$ and $y$.

We can find $w, z \in A$ such that $x = f w$, and $y = f z$.

If $f$ is a unit, then $f \mathop \backslash d$ by definition.

Suppose $f \nmid d$.

Then the factorization of $f$ must contain an irreducible element, call this $g$, that does not divide $d$.

But then $g$ must divide some $x_j$, $j > t$ and $g$ must divide some $y_k$ where $k > t$.

Either $g$ is a unit, contradicting its irreducibility, or $x_j$ and $y_k$ are reducible, which is a contradiction also.

Hence:
 * $f \mathop \backslash d$

and so $x$ and $y$ have a greatest common divisor.