Euclidean Algorithm and Extended Euclidean Algorithm

Euclidean Algorithm

This algorithm allows you to find the greatest common factor (or greatest common divisor) of two numbers.

To do the Euclidean Algorithm, start with the larger number, $a$, and write it as $a=\left(b\right)\left(c\right)+d$, where $b$ is the smaller number, $c$ is as large as possible, and then $d$ is the remainder. Then, on the next line, write an equation in the same form except $a$ is replaced by $b$ and $b$ is replaced by $d$. Continue this way until $d=0$, and the last non-zero value of $d$ is the greatest common divisor.

Example

$gcd(132,473)$:

$473=\left(132\right)\left(3\right)+77$

$132=\left(77\right)\left(1\right)+55$

$77=\left(55\right)\left(1\right)+22$

$55=\left(22\right)\left(2\right)+11$

$22=\left(11\right)\left(2\right)+0$

The last non-zero remainder is the GCD, so that is 11 in this case.

Extended Euclidean Algorithm

The Extended Euclidean algorithm is used to calculate the multiplicative inverse of a number. This means finding a number $y$ for $x$ such that $xy\mathrm{mod}n=1$. This can only happen if numbers $x$ and $y$ are coprime, meaning they share no factors other than 1.

Example

To begin, you need to do the standard Euclidean algorithm:

to be continued...