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