  We investigate the consistency relation relating the squeezed limit of the
bispectrum to the scalar spectral index in single field models of inflation. We
give a simple integral formula for the bispectrum in the squeezed limit in
terms of the free mode mode functions of the primordial curvature perturbation,
in any Lorentz invariant single field model of inflation and without resorting
to any approximation, generalizing a recent result obtained by Ganc and Komatsu
in the case of canonical kinetic terms. We use our result to verify the
consistency relation in an exactly solvable class of models with a non-trivial
speed of sound. We then verify the consistency relation at the first
non-trivial order in the slow-varying approximation in general single field
inflation (a known result) and at second order in this approximation in
canonical single field inflation.
