http://projecteuler.net/index.php?section=problems&id=3

euler-003

The prime factors of 13195 are 5, 7, 13 and 29.

What is the largest prime factor of the number 600851475143 ?

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$ perl -e '$x = 600851475143; for $i (2..sqrt($x)) { print "$i\n" if $x % $i == 0; }'
71
839
1471
6857
59569
104441
486847

$ perl -e '$x = 486847; for $i (2..sqrt($x)) { print "$i\n" if $x % $i == 0; }'
71
$ perl -e '$x = 104441; for $i (2..sqrt($x)) { print "$i\n" if $x % $i == 0; }'
71
$ perl -e '$x = 59569; for $i (2..sqrt($x)) { print "$i\n" if $x % $i == 0; }'
71
$ perl -e '$x = 6857; for $i (2..sqrt($x)) { print "$i\n" if $x % $i == 0; }'
(nothing)

Answer: 6857