def _compute_error(self):
        """ Evaluate the absolute error of the Nystroem approximation for each column """
        # err_i = sum_j R_{k,ij} A_{k,ji} - d_i
        self._err = np.sum(np.multiply(self._R_k, self._C_k.T), axis=0) - self._d
Evaluate the absolute error of the Nystroem approximation for each column