code: def _get_mu_tensor(self):
      
      root = self._get_cubic_root()
      dr = self._h_max / self._h_min
      mu = tf.maximum(
          root**2, ((tf.sqrt(dr) - 1) / (tf.sqrt(dr) + 1))**2)
      return mu
docs: Get the min mu which minimize the surrogate.
  
      Returns:
        The mu_t.