Euclidean Algorithm

Objective

The Euclidean Algorithm efficiently computes the Greatest Common Divisor (GCD) of two integers $A$ and $B$.

The core idea is based on the following identity:

$$\gcd(A, B) = \gcd(B, A \bmod B)$$

Code

def gcd(a, b):
    if b == 0:
        return a
    else:
        return gcd(b, a % b)

Proof

Let $G = \gcd(A, B)$. Then, we can write $A=aG$, $B=bG$, where $a$ and $b$ are coprime integers.

Dividing $A$ by $B$ gives:

$$A = Bq + R$$

Then:

$$R = A - Bq = (a - bq)G = rG$$

So $R = rG$, and we want to prove that $\gcd(B, R) = G$ is true if and only if $b$ and $r$ are coprime.

Suppose, for contradiction, that $b$ and $r$ are not coprime. Then there exists a common divisor $m > 1$ such that:

$$b = mb', \quad r = mr'$$

where $b'$ and $r'$ are coprime. Then:

$$\begin{align*} a &= bq + r \\ &= mb'q + mr' \\ &= m(b'q + r') \end{align*}$$

This means $a$ is also divisible by $m$, implying $a$ and $b$ share a common factor $m$, contradicting the assumption that $a$ and $b$ are coprime. Thus, $b$ and $r$ must be coprime.

Proving the reverse is trivial, so we don't prove it here.

Written by ryan Kim from A41