def isPrime(n): for i in range(2, n // 2): if n % i == 0: return False return True def primenumbers(): # yield 1 yield 2 yield 3 i = 4 while True: if isPrime(i): yield i i += 1 def main(): target = 600851475143 primefactors = [] print(primefactors, target) while target > 1: for p in primenumbers(): if target % p == 0: primefactors.append(p) target = target // p print(primefactors, target) break print(sorted(primefactors)[-1]) main()