def _f_A20(self, r_a, r_s):
        """
        equation A20 in Eliasdottir (2013)
        :param r_a: r/Ra
        :param r_s: r/Rs
        :return:
        """
        return r_a/(1+np.sqrt(1 + r_a**2)) - r_s/(1+np.sqrt(1 + r_s**2))