```
def biggest_factor(number):
factor = 2
while not (number % factor) and number > factor:
number /= factor
factor = 3
limit = factor * factor
while limit <= number:
while not (number % factor) and number > factor:
number /= factor
#print factor,number
limit += 4*(factor+1)
factor += 2
return number
print biggest_factor(600851475143)
```

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