code: def styblinski_tang(theta):
      
      x, y = theta
      obj = 0.5 * (x ** 4 - 16 * x ** 2 + 5 * x + y ** 4 - 16 * y ** 2 + 5 * y)
      grad = np.array([
          2 * x ** 3 - 16 * x + 2.5,
          2 * y ** 3 - 16 * y + 2.5,
      ])
      return obj, grad
docs: Styblinski-Tang function