public static function pentagonalNumber(int $n) : array { $pentagonal = []; // Bad input; return empty list if ($n <= 0) { return $pentagonal; } // Standard case for pn: (3n² - n) / 2 for ($i = 1; $i <= $n; $i++) { $pentagonal[$i] = (3 * $i ** 2 - $i) / 2; } return $pentagonal; }