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, whilst 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. 
Anthony W. Brookfield, Carsten van de Bruck, Lisa M.H. Hall