The generalized second law of thermodynamics for interacting $f(R)$   gravity
  We examine the validity of the generalized second law (GSL) of gravitational thermodynamics in the context of interacting $f(R)$ gravity. We take into account that the boundary of the universe to be confined by the dynamical apparent horizon in a flat FRW universe. We study the effective equation of state, deceleration parameter and GSL in this interaction-framework. We find that the evolution of the total entropy increases through the interaction term. As a example, we consider a $f(R)$ gravity with a power-law dependence on the curvature $R$. Here, we find exact solutions for a model in which the interaction term is related to the total energy density of matter. 
Ramon Herrera and Nelson Videla
gr-qc