def _alpha_eff(self, r_eff, n_sersic, k_eff):
        """
        deflection angle at r_eff
        :param r_eff:
        :param n_sersic:
        :param k_eff:
        :return:
        """
        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
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
deflection angle at r_eff
        :param r_eff:
        :param n_sersic:
        :param k_eff:
        :return: