x and y are relatively prime : <=> gcd(x, y) = 1.
x is prime : <=> x > 1 /\ (forall y: y | x => (y = 1 \/ y = x)).
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, ...