# Integers Divided by GCD are Coprime

## Theorem

Let $a, b \in \Z$ be integers which are not both zero.

Let $d$ be a common divisor of $a$ and $b$, that is:

$\dfrac a d, \dfrac b d \in \Z$

Then:

$\gcd \set {a, b} = d$
$\gcd \set {\dfrac a d, \dfrac b d} = 1$

that is:

$\dfrac a {\gcd \set {a, b} } \perp \dfrac b {\gcd \set {a, b} }$

where:

$\gcd$ denotes greatest common divisor
$\perp$ denotes coprimality.

## Proof 1

Let $d = \gcd \set {a, b}$.

By definition of divisor:

$d \divides a \iff \exists s \in \Z: a = d s$
$d \divides b \iff \exists t \in \Z: b = d t$

So:

 $\ds \exists m, n \in \Z: \,$ $\ds d$ $=$ $\ds m a + n b$ Bézout's Identity $\ds \leadstoandfrom \ \$ $\ds d$ $=$ $\ds m d s + n d t$ Definition of $s$ and $t$ $\ds \leadstoandfrom \ \$ $\ds 1$ $=$ $\ds m s + n t$ dividing through by $d$ $\ds \leadstoandfrom \ \$ $\ds \gcd \set {s, t}$ $=$ $\ds 1$ Bézout's Identity $\ds \leadstoandfrom \ \$ $\ds \gcd \set {\frac a d, \frac b d}$ $=$ $\ds 1$ Definition of $s$ and $t$

$\blacksquare$

## Proof 2

Let $d = \gcd \set {a, b}$.

We have:

$(1): d \divides a \iff \exists s \in \Z: a = d s$
$(2): d \divides b \iff \exists t \in \Z: b = d t$

We have to prove:

$\gcd \set {s, t} = 1$

Aiming for a contradiction, suppose $\gcd \set {s, t} \ne 1$.

So:

$(3): \exists k \in \N \setminus \set 1$ such that $k \divides s \land k \divides t$

So:

$(4): \exists m, n \in \N: s = k m, t = k n$

Substituting from $(4)$ in $(1)$ and $(2)$:

$a = d k m$, $b = d k n$

Therefore:

$d k \divides a \land d k \divides b$

From $(3)$ we have:

 $\ds$  $\ds k \in \N \land k \ne 1$ $\ds \leadsto \ \$ $\ds$  $\ds k > 1$ $\ds \leadsto \ \$ $\ds$  $\ds d k > d$

As $d k$ is a common divisor of $a$ and $b$ greater than $d$, this contradicts $d = \gcd \set {a, b}$.

So our initial assumption that $\gcd \set {s, t} \ne 1$ is false.

Therefore, from Proof by Contradiction, we have:

$\gcd \set {s, t} = 1 \implies \gcd \set {\dfrac a d, \dfrac b d} = 1$

$\blacksquare$

## Proof 3

Because $d$ is a common divisor of $a$ and $b$, we may form the expressions:

$a = d r$
$b = d s$

where $r, s \in \Z$.

Then:

 $\ds d$ $=$ $\ds \gcd \set {a, b}$ by hypothesis $\ds$ $=$ $\ds \gcd \set {d r, d s}$ $\ds$ $=$ $\ds d \gcd \set {r, s}$ GCD of Integers with Common Divisor $\ds \leadstoandfrom \ \$ $\ds 1$ $=$ $\ds \gcd \set {r, s}$ dividing through by $d$ $\ds$ $=$ $\ds \gcd \set {\dfrac a d, \dfrac b d}$ Definition of $r$ and $s$

$\blacksquare$

## Also presented as

It can be expressed so as not to include fractions:

$\gcd \set {a, b} = d \iff \exists s, t \in \Z: a = d s \land b = d t \land \gcd \set {s, t} = 1$