Viability of f(R) theories with additional powers of curvature

We consider a modified gravity theory, f(R)=R-a/R-n+bR(m), in the metric formulation, which has been suggested to produce late-time acceleration in the Universe, while satisfying local fifth-force constraints. We investigate the parameter range for this theory, considering the regimes of early and late-time acceleration, big bang nucleosynthesis, and fifth-force constraints. We conclude that it is difficult to find a unique range of parameters for consistency of this theory.