Bézout's Lemma/Euclidean Domain

Theorem
Let $\struct {D, +, \times}$ be a Euclidean domain whose zero is $0$ and whose unity is $1$.

Let $\nu: D \setminus \set 0 \to \N$ be the Euclidean valuation on $D$.

Let $a, b \in D$ such that $a$ and $b$ are not both equal to $0$.

Let $\gcd \set {a, b}$ be the greatest common divisor of $a$ and $b$.

Then:
 * $\exists x, y \in D: a \times x + b \times y = \gcd \set {a, b}$

such that $\gcd \set {a, b}$ is the element of $D$ such that:
 * $\forall c = a \times x + b \times y \in D: \map \nu {\gcd \set {a, b} } \le \map \nu c$

Proof
We are given that $a, b \in D$ such that $a$ and $b$ are not both equal to $0$.

, suppose specifically that $b \ne 0$.

Let $S \subseteq D$ be the set defined as:


 * $S = \set {x \in D_{\ne 0}: x = m \times a + n \times b: m, n \in D}$

where $D_{\ne 0}$ denotes $D \setminus 0$.

Setting $m = 0$ and $n = 1$, for example, it is noted that $b \in S$.

Therefore $S \ne \O$.

By definition, $\nu$ has the properties:
 * $(1): \quad \forall a, b \in D, b \ne 0: \exists q, r \in D$ such that $\map \nu r < \map \nu b$, or $r = 0$, such that:
 * $a = q \times b + r$
 * $(2): \quad \forall a, b \in D, b \ne 0$:
 * $\map \nu a \le \map \nu {a \times b}$

Let $\nu \sqbrk S$ denote the image of $S$ under $\nu$.

We have that:
 * $\nu \sqbrk S \subseteq \N$

Hence by the Well-Ordering Principle $\nu \sqbrk S$ has a smallest element.

Let $d \in S$ be such that $\map \nu d$ is that smallest element of $\nu \sqbrk S$.

By definition of $S$, we have that:
 * $d = u \times a + v \times b$

for some $u, v \in D$.

Let $x \in S$.

By $(2)$ above:
 * $x = q \times d + r$

such that either:
 * $\map \nu r < \map \nu d$

or:
 * $r = 0$

$r \ne 0$.

Then:

which contradicts the choice of $d$ as the element of $S$ such that $\map \nu d$ is the smallest element of $\nu \sqbrk S$.

Therefore:
 * $\forall x \in S: x = q \times d$

for some $q \in D$.

That is:
 * $\forall x \in S: d \divides x$

where $\divides$ denotes divisibility.

In particular:


 * $d \divides a = 1 \times a + 0 \times b$


 * $d \divides b = 0 \times a + 1 \times b$

Thus:
 * $d \divides a \land d \divides b \implies \map \nu 1 \le \map \nu d \le \map \nu {\gcd \set {a, b} }$

However, note that as $\gcd \set {a, b}$ also divides $a$ and $b$ (by definition), we have:

Since $d$ is the element of $S$ such that $\map \nu d$ is the smallest element of $\nu \sqbrk S$:
 * $\gcd \set {a, b} = d = u \times a + v \times b$