Higher derivative corrections of $f(R)$ gravity with varying equation of   state in the case of variable $G$ and $\Lambda$
  In this paper, we study three models of f(R) modified gravity including higher order terms based on different equation of state parameter. We also assume variable G and $\Lambda$. By using numerical analysis, we obtain the behavior of some important cosmological parameters, like the Hubble expansion H parameter and the deceleration parameter q. The statefinder diagnostics is also performed for all models. 
M. Khurshudyan, B. Pourhassan, A. Pasqua
gr-qc