Math if fun

Greatest Common Divisor, gcd(a, b)
- If b|a then gcd(a, b) = b.
- If a = bt + r, for integers t and r, then gcd(a, b) = gcd(b, r).
  - Indeed, every common divisor of a and b also divides r. Thus gcd(a, b) divides r. But, of course,gcd(a, b)|b. Therefore, gcd(a, b) is a common divisor of b and r and hence gcd(a, b) ≤ gcd(b, r). The reverse is also true because every divisor of b and r also divides a.
```
 1 /**
 2  * Java method to calculate the greatest common divisor
 3  * @return the gcd of two integer
 4  */
 5 private static int gcd(int x, int y) {
 6     if(y == 0) {
 7         return x;
 8     }
 9     return gcd(y, x % y);
10 }
```
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