Bezout’s lemma

Lemma 28.1(Lemma).

Let $a$ and $b$ be integers with gcd $d$. Then, there exist integers $x$ and $y$ such that $ax+by=d$. Moreover, all integers of the type $an+bm$ are multiples of $d$.

Proof.

Given any non-zero integers$a$ and $b$, let $S=\{ax+by:x,y\in \mathbb{Z},ax+by>0\}$. $S$ has at least one element, since either $a$ or $-a$ is in $S$. Since $S$ is bounded below, $S$ must have a minimum element. Let $d$ be this minimum element. We have to show that $d$ divides $a$ and $b$, and that all common factors of $a$ and $b$ are $\le d$.

The euclidean division of $a$ by $d$ may be written as $a=qd+r$, where $0\le r<d$. Since $d\in S$, $d$ can be written as a linear combination of $a$ and $b$, hence $r$ can be written as a linear combination of $a$ and $b$. Since $r$ cannot be in $S$ ($d$ is smallest element), $r$ is forced to be $0$. Thus, $d$ divides $a$, and similarly, $b$.

Now consider a common factor $c$ of $a$ and $b$. We have

$$\begin{array}{rl}d& =ua+vb\\ & =uc{a}^{\prime }+vc{b}^{\prime }\\ & =c(u{a}^{\prime }+v{b}^{\prime })\end{array}$$

That is, $c$ is a divisor of $d$. Since $d>0$, we have $c\le d$.□