The prime factors of 13195 are 5, 7, 13 and 29.
What is the largest prime factor of the number 600851475143 ?
I just wrote a quick-and-dirty search for the smallest prime factor of a number that tries 2 and 3 and then searches by trying one less than a multiple of 6 and one more than a multiple of 6.
I then repeatedly divide my goal by its smallest prime factor until I end up with a prime number and that's the answer.