A note on viability of nonminimally coupled $f(R)$ theory
  Consistency conditions for nonminimally coupled $f(R)$ theories have been derived by requiring the absence of tachyons and instabilities in the scalar fluctuations. This note confirms these results and clarifies a subtlety regarding different definitions of sound speeds. 
Tomi Koivisto, Nicola Tamanini
gr-qc