Left: relative deviation induced by f(R) gravity and massive neutrinos in the matter power spectrum measured in a subset of our simulations at z = 0. The large deviation caused by the additional growth in |f_R0| = 10⁻⁴ is almost completely counteracted by massive neutrinos with ∑m_ν = 0.3 eV. We find a similar case for |f_R0| = 10⁻⁵ and ∑m_ν = 0.15 eV. Right: same degeneracy in the simulated abundance of halos at z = 0. We note that the degeneracy is non-trivial; the same P(k) can lead to different cluster abundances in f(R) since the collapse threshold is changed in MG. The uncertainty for the cluster abundance is calculated with Poisson error bars. Shaded grey bands indicate the 10% deviation region in both plots.