δN versus covariant perturbative approach to non-Gaussianity outside the horizon in multifield inflation

We compute the super-Hubble evolution of non-Gaussianity of primordial curvature perturbations in two-field inflation models by employing two formalisms: delta N and covariant formalisms. Although the two formalisms treat the evolution of fluctuations radically different, we show that the formulas of the f(NL) parameter agree quantitatively with each other within 1% accuracy. We analytically find that the amplitude of f(NL) decays no faster than a(-3) as the inflationary trajectory reaches to the adiabatic limit for generic potentials.