On the redundancy of operators and the bispectrum in the most general
  second-order scalar-tensor theory
  In this short note we explain how to use the linear equation of motions to
simplify the third-order action for the cosmological fluctuations. No field
redefinition is needed in this exact procedure which considerably limits the
range of independent cubic operators, and hence of possible shapes of the
primordial bispectrum. We demonstrate this in the context of the most general
single-field scalar-tensor theory with second-order equations of motion, whose
third-order action has been calculated recently in arXiv:1107.2642 and
1107.3917. In particular, we show that the three cubic operators initially
pointed out in these works as new compared to k-inflation can actually be
expressed in terms of standard k-inflationary operators.
