def primes():
"""
yields prime numbers
"""
P = (2,)
yield 2
for i in count(3, 2):
if all(i % p for p in upto(sqrt(i), P)):
P = P + (i,)
yield i
def largest_prime_factor(n):
'''
returns the largest prime factor to n
'''
return max(filter(lambda p: not n % p, upto(sqrt(n), primes())))
