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