2.4 The Euclidean Algorithm

2.4.1 The Euclidean Algorithm $(a \le b < 0)$

Lemma (will be proved)

$a =qb + r \Rightarrow gcd(a,b) = gcd(b,r)$

Proof of $gcd(a,b) = r_{n}$

pf

example

숫자가 클수록 모든 positive divisor을 구하기는 어렵다

정리

• prove the Euclidean Algorithm
• We can compute the gcd(a,b) using Euclidean Algorithm (even for large a and b)

2.4.2 Find $x,y \in \mathbb{Z} \text{ with } ax+by=gcd(a,b)$

유클리드 호제법의 응용

답중의 하나를 위의 방법으로 구할 수 있다.

Theorem 2.7

Euclidean Algorithm 이용해서 증명함

정리

• We can find $x,y \in \mathbb{Z} \text{ s.t. } ax+by=gcd(a,b)$ using the Euclidean Algorithm

2.4.3 The Least Common Multiple

Definition 2.4

Given $a,b \in \mathbb{Z} \left( a \neq 0 \text{ or } b \neq 0 \right)$, the least common multiple of $\text{a and b}$ is the positive integer m satisfying

(a) $a \mid m$ and $b \mid m$ (common multiple)
(b) $a \mid c$ and $b \mid c \Rightarrow m \le c$ (least)

In this case, we write $m = lcm(a,b$)

Theorem 2.8

$gcd(a,b) lcm(a,b) = ab$

Corollary 2.8

For any $a,b \in \mathbb{N}, lcm(a,b)=ab \Longleftrightarrow gcd(a,b) = 1$

Proof of Theorem 2.8

정리

• We can write the definition of $lcm(a,b)$
• We can prove the relation between $gcd(a,b) \text{ and } lcm(a,b)$