public static function hexagonalNumber(int $n) : array { $hexagonal = []; // Bad input; return empty list if ($n <= 0) { return $hexagonal; } // Standard case for hn: (2n × (2n - 1)) / 2 for ($i = 1; $i <= $n; $i++) { $hexagonal[$i] = 2 * $i * (2 * $i - 1) / 2; } return $hexagonal; }