GCD

public static int getGcd(int a,int b){
  int temp;
  while(b!=0){
   temp = a%b;
   a=b;
   b=temp;
  }  
  return a;
 }

Recursive approach :

public static int getGCDRecursively(int a,int b){
  if(b==0) return a;
  else if(b>a) return getGCDRecursively(b,a);
  else return getGCDRecursively(b, a%b);
 }

Co-prime numbers : Two numbers are called relatively prime, or coprime if their greatest common divisor equals 1. For example, 9 and 28 are relatively prime.