Singularity phenomena in viable f(R) gravity
  The curvature singularity in viable f(R) gravity models is examined when the background density is dense. This singularity could be eliminated by adding the $R^{2}$ term in the Lagrangian. Some of cosmological consequences, in particular the source for the scalar mode of gravitational waves, are discussed. 
Chung-Chi Lee, Chao-Qiang Geng and Louis Yang
astro-ph.CO gr-qc