code: def _alpha_eff(self, r_eff, n_sersic, k_eff):
          
          b = self.b_n(n_sersic)
          alpha_eff = n_sersic * r_eff * k_eff * b**(-2*n_sersic) * np.exp(b) * special.gamma(2*n_sersic)
          return -alpha_eff
docs: deflection angle at r_eff
          :param r_eff:
          :param n_sersic:
          :param k_eff:
          :return: