1.0 1.2 1.4 1.6 1.8 2.0 0 1 2 3 4 1/ z = 0.174 h Mpc -1 n=2  =0.95 n=2  =8.0 FIG. 7: The evolution of 1 /η = Φ / Ψ with n = 2. different choices of the model parameters when the current matter density parameter is fixed to Ω$_{m}$$_{0}$ = 0 . 27 and n ≥ 2. This variation is smaller than that caused by the uncertainty of Ω$_{m}$$_{0}$[33]. So, at present it does not help much to single out the best DE model among the considered ones, in contrast to the f ( R ) DE model[12] (it has the same behaviour (15) for R ≫ R$_{s}$ ) in the case corresponding to n = 0 . 5 in our notations which was recently studied in Ref. [40]. Finally we consider the quantity 1 /η = Φ / Ψ, namely the ratio of gravitational potential to curvature perturbation, for which some results from observational data were recently obtained in Ref. [4]. In f ( R ) gravity, 1 /η is expressed as Due to the stability conditions F > 0 , F ′ > 0, this quantity always lies between 1 and 2. Thus, stable f ( R ) DE models may not explain such a large value of 1 /η which is presented in Ref. [4] for the redshift interval 1 < z < 2. Figure 7 shows the evolution of 1 /η for n = 2 and λ = 0 . 95 (the minimal possible value) and 8. 1 η = 2 − 1 1 + 2 k 2 a 2 F ′ F . (33) 15