# Difference between revisions of "Euclidean algorithm"

 This is an AoPSWiki Word of the Week for Jan 10-17

The Euclidean algorithm (also known as the Euclidean division algorithm or Euclid's algorithm) is an algorithm that finds the greatest common divisor (GCD) of two elements of a Euclidean domain, the most common of which is the nonnegative integers $\mathbb{Z}_{\geq 0}$, without factoring them.

## Main idea and informal description

The basic idea is to repeatedly use the fact that $\gcd({a,b}) \equiv \gcd({a,a - b})$

If we have two non-negative integers $a,b$ with $a\ge b$ and $b=0$, then the greatest common divisor is ${a}$. If $a\ge b>0$, then the set of common divisors of ${a}$ and $b$ is the same as the set of common divisors of $b$ and $r$ where $r$ is the remainder of division of ${a}$ by $b$. Indeed, we have $a=mb+r$ with some integer$m$, so, if ${d}$ divides both ${a}$ and $b$, it must divide both ${a}$ and $mb$ and, thereby, their difference $r$. Similarly, if ${d}$ divides both $b$ and $r$, it should divide ${a}$ as well. Thus, the greatest common divisors of ${a}$ and $b$ and of $b$ and $r$ coincide: $GCD(a,b)=GCD(b,r)$. But the pair $(b,r)$ consists of smaller numbers than the pair $(a,b)$! So, we reduced our task to a simpler one. And we can do this reduction again and again until the smaller number becomes $0$

## General Form

Start with any two elements $a$ and $b$ of a Euclidean Domain

• If $b=0$, then $\gcd(a,b)=a$.
• Otherwise take the remainder when ${a}$ is divided by $a (\bmod {b})$, and find $\gcd(a,a \bmod {b})$.
• Repeat this until the remainder is 0.

$a (\bmod b) \equiv r_1$
$b (\bmod r_1) \equiv r_2$
$\vdots$
$r_{n-1} (\bmod r_n) \equiv 0$
Then $\gcd({a,b}) = r_n$

Usually the Euclidean algorithm is written down just as a chain of divisions with remainder:

for $r_{k+1} < r_k < r_{k-1}$
$a = b \cdot q_1+r_1$
$b = r_1 \cdot q_2 + r_2$
$r_1 = r_2 \cdot q_3 + r_3$
$\vdots$
$r_{n-1} = r_n \cdot q_{n+2} +0$
and so $\gcd({a,b}) = r_n$

## Simple Example

To see how it works, just take an example. Say $a=112,b=42$. We have $112\equiv 28\pmod {42}$, so ${\gcd(112,42)}=\gcd(42,28)$. Similarly, $42\equiv 14\pmod {28}$, so $\gcd(42,28)=\gcd(28,14)$. Then $28\equiv {0}\pmod {14}$, so ${\gcd(28,14)}={\gcd(14,0)} = 14$. Thus $\gcd(112,42)=14$.

• ${112 = 2 \cdot 42 + 28 \qquad (1)}$
• $42 = 1\cdot 28+14\qquad (2)$
• $28 = 2\cdot 14+0\qquad (3)$

## Linear Representation

An added bonus of the Euclidean algorithm is the "linear representation" of the greatest common divisor. This allows us to write $\gcd(a,b)=ax+by$, where $x,y$ are some elements from the same Euclidean Domain as $a$ and $b$ that can be determined using the algorithm. We can work backwards from whichever step is the most convenient.

In the previous example, we can work backwards from equation $(2)$:

$14 = 42-1\cdot 28$
$14 = 42-1\cdot (112-2\cdot 42)$
$14 = 3\cdot 42-1\cdot 112.$