Project Euler.net Answers

Problem 3

Solved on 2/29/2008

The prime factors of 13195 are 5, 7, 13 and 29. What is the largest prime factor of the number 600851475143 ? Note: This problem has been changed recently, please check that you are using the right number.

$number = 600851475143;
for($ic = 2; $ic <= sqrt($number); $ic++){
  $thisint = $number / $ic;
  $string = strval($thisint);
  $isint = strpos($string, '.');
  if($isint === FALSE){
    $bn = $number / $ic;
    if($this->isPrime($bn)){
      $answer = $bn;
      $ic = $number;
    }else if(isPrime($ic)){
      $answer = $ic;
    }
  }
}


function isPrime($val){
  if(($val % 2 ) == 0) return FALSE;
  for($c = 3; $c <= sqrt($val); $c+=2) {
    if(($val % $c ) == 0) return FALSE;
  }
  return TRUE;
}
$answer = 6,857