# Euclidean Algorithm/Proof 2

## Algorithm

The Euclidean algorithm is a method for finding the greatest common divisor (GCD) of two integers $a$ and $b$.

Let $a, b \in \Z$ and $a \ne 0 \lor b \ne 0$.

The steps are:

$(1): \quad$ Start with $\tuple {a, b}$ such that $\size a \ge \size b$. If $b = 0$ then the task is complete and the GCD is $a$.
$(2): \quad$ If $b \ne 0$ then you take the remainder $r$ of $\dfrac a b$.
$(3): \quad$ Set $a \gets b, b \gets r$ (and thus $\size a \ge \size b$ again).
$(4): \quad$ Repeat these steps until $b = 0$.

Thus the GCD of $a$ and $b$ is the value of the variable $a$ after the termination of the algorithm.

## Proof

Suppose $a, b \in \Z$ and $a \lor b \ne 0$.

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

By definition of common divisor:

$d \divides a$

and:

$d \divides b$

Hence from Common Divisor Divides Integer Combination:

$d \divides \paren {a - q b}$

That is:

$d \divides r$

Thus $d$ is a common divisor of $b$ and $r$.

Let $c$ be an arbitrary common divisor of $b$ and $r$.

Then:

$c \divides \paren {q b + r}$

That is:

$c \divides a$

Thus $c$ is a common divisor of $a$ and $b$.

Hence by definition of GCD:

$c \le d$

Hence, again by definition of GCD: $d = \gcd \set {b, r}$

Then we work down the system of equations:

 $\ds \gcd \set {a, b}$ $=$ $\ds \gcd \set {b, r_1}$ $\ds$ $=$ $\ds \gcd \set {r_1, r_2}$ $\ds$ $=$ $\ds \cdots$ $\ds$ $=$ $\ds \gcd \set {r_{n - 1}, r_n}$ $\ds$ $=$ $\ds \gcd \set {r_n, 0}$ $\ds$ $=$ $\ds r_n$

$\blacksquare$

## Source of Name

This entry was named for Euclid.