code: 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))
docs: equation A20 in Eliasdottir (2013)
          :param r_a: r/Ra
          :param r_s: r/Rs
          :return: