euler3.py

from gmpy import is_prime
from math import sqrt

def largest_prime_factor(n):
    for i in xrange(int(sqrt(n)), 1, -1):
        if (n%i == 0) and is_prime(i, 100):
            return i

def main():
    print largest_prime_factor(600851475143)

if __name__ == "__main__":
    main()