Squeezed primordial bispectrum from general vacuum state
  We study the general relation between the power spectrum and the squeezed
limit of the bispectrum of the comoving curvature perturbation produced during
single-field slow-roll inflation when the initial state is a general vacuum.
Assuming the scale invariance of the power spectrum, we derive a formula for
the squeezed limit of the bispectrum, represented by the parameter fNL, which
is not slow-roll suppressed and is found to contain a single free parameter for
a given amplitude of the power spectrum. Then we derive the conditions for
achieving a scale-invariant fNL, and discuss a few examples.
