Viability of nonminimally coupled f (R) gravity
  In this work we explore the viability of nonminimally coupled matter- curvature gravity theories, namely the conditions required for the absence of tachyon instabilities and ghost degrees of freedom. We contrast our finds with recent claims of a pathological behaviour of this class of models, which resorted to, in our view, an incorrect analogy with k-essence. 
Orfeu Bertolami and Jorge P\'aramos
gr-qc