0 is written in front
This article is inspired by NaVi_Awson , and even quoted directly in some places, which is explained here.
1 Number Theory
1.0 gcd
1.0.0 gcd
$gcd(a,b) = gcd(b,a\;mod\;b)$
Proof: Set $c\mid a$, $c\mid b$, then $c\mid (ba)$.
Let $c\nmid a$, then $c$ is not the common factor of $a,ba$.
Let $c\mid a$, $c\nmid b$, then $c$ is not the common factor of $a,ba$.
int gcd(int a,int b){ if(!b) return a; return gcd(b,a%b); }