<?php function greatest_prime($n) { for($x1 = (int)floor(sqrt($n)); $x1 > 3; $x1--) { if($n % $x1 != 0) continue; $x2 = $n/$x1; if(is_prime($x1)) return $x1; if(is_prime($x2)) return $x2; } } function is_prime($x) { for($y = (int)floor(sqrt($x)); $y > 1; $y--) if($x % $y == 0) return false; return true; } var_dump(greatest_prime(600851475143));
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