code: def mean_second_derivative_central(x):
      
  
      diff = (_roll(x, 1) - 2 * np.array(x) + _roll(x, -1)) / 2.0
      return np.mean(diff[1:-1])
docs: Returns the mean value of a central approximation of the second derivative
  
      .. math::
  
          \\frac{1}{n} \\sum_{i=1,\ldots, n-1}  \\frac{1}{2} (x_{i+2} - 2 \\cdot x_{i+1} + x_i)
  
      :param x: the time series to calculate the feature of
      :type x: numpy.ndarray
      :return: the value of this feature
      :return type: float