docs: def abs_energy(x):
      """
      Returns the absolute energy of the time series which is the sum over the squared values
  
      .. math::
  
          E = \\sum_{i=1,\ldots, n} x_i^2
  
      :param x: the time series to calculate the feature of
      :type x: numpy.ndarray
      :return: the value of this feature
      :return type: float
      """
      if not isinstance(x, (np.ndarray, pd.Series)):
          x = np.asarray(x)
      return np.dot(x, x)