___________GCD(X,Y) = GCD(X,Y - X)___________

X is fibonacci if 5x^2-4 and 5x^2+4 is square

X is prime if 2^(x-1)%x == 1 except 2

Xor from 1 to n

• n%4 == 0 return n
• n%4 == 1 return 1
• n%4 == 2 return n+1
• n%4 == 3 return 0