A, gcd
GCD (A, B) = GCD (B, A)
GCD (A, B) = GCD (-a, B)
GCD (A, B) = GCD (| A |, | B |)
GCD (A, 0) = | A |
GCD (A, K * A) = | A |
GCD (A * n-, B * n-) = n-* GCD (A, B)
if d | a and d | b, then d | gcd (a , b)
if n | ab and gcd (a, n) = 1 , then n | b
if gcd (a, p) = 1 and gcd (b, p) = 1 , then the gcd (a * b, p) = 1