function is_prime($n) {
if ($n == 2) {
return true;
}
if ($n < 2 || $n % 2 == 0) {
return false;
}
for ($i = 3; $i * $i <= $n; $i += 2) {
if ($n % $i == 0) {
return false;
}
}
return true;
}
function cyclically_rotate_left($n) {
$m = $n;
$p = 1;
while ($m >= 10) {
$p *= 10;
$m /= 10;
}
return (int)($m + 10 * ($n % $p));
}
function is_circular_prime($n) {
if (!is_prime($n)) {
return false;
}
$rotated_n = cyclically_rotate_left($n);
while ($rotated_n != $n) {
if ($rotated_n < $n || !is_prime($rotated_n)) {
return false;
}
$rotated_n = cyclically_rotate_left($rotated_n);
}
return true;
}
$n = 2;
for ($i = 0; $i < 18; $n++) {
if (is_circular_prime($n)) {
if ($i > 0) {
echo ", ";
}
printf("%d",$n);
$i++;
}
}
/*
run:
2, 3, 5, 7, 11, 13, 17, 37, 79, 113, 197, 199, 337, 1193, 3779, 11939, 19937, 193939
*/
