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In [1]:

    
!factor 600851475143 | rev | awk '{print $1}' | rev



In [2]:

    
import math

def largest_factor(x):
    factors = []
    factor = 2
    while factor <= math.sqrt(x):
        while x % factor == 0:
            factors.append(factor)
            x /= factor
        factor += 1
    if x != 1:
        factors.append(x)
    return factors



In [3]:

    
%timeit largest_factor(600851475143)









    



1000 loops, best of 3: 1.31 ms per loop



In [4]:

    
def largest_factor(x):
    factors = []
    factor = 2
    while factor * factor <= x:
        while x % factor == 0:
            factors.append(factor)
            x /= factor
        factor += 1
    if x != 1:
        factors.append(x)
    return factors



In [5]:

    
%timeit largest_factor(600851475143)









    



1000 loops, best of 3: 990 us per loop



In [6]:

    
largest_factor(1000)









    Out[6]:





[2, 2, 2, 5, 5, 5]



In [7]:

    
largest_factor(600851475143)









    Out[7]:





[71, 839, 1471, 6857L]



In [8]:

    
largest_factor(600851475143)[-1]









    Out[8]:





6857L