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

What is the largest prime factor of the number 600851475143 ?

**Script**

Execution Time: 18.15 seconds

function isPrime($n) { for ($x=2; $x<=floor($n/2); $x++) { if ($n%$x == 0) { return false; } } return true; } $comp = 600851475143; for ($x = 3; $x <= ceil($comp/2); $x = $x+2) { if ($comp%$x == 0) { $y = $comp / $x; if (isPrime($y)) { echo $y.PHP_EOL; break; } } }

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I believe you have the wrong solution here?

This solution is for problem #2.

You are correct. Somehow I copied the wrong one in. It has been fixed now and shows the correct solution. Thanks for pointing it out.