  In single-field inflation, violation of the slow-roll approximation can lead
to growth of curvature perturbation outside the horizon. This violation is
characterized by a period with a large negative value of the second slow-roll
parameter. At an early time, inflation must satisfy the slow-roll
approximation, so the large-scale curvature perturbation can explain the cosmic
microwave background fluctuations. At intermediate time, it is viable to have a
theory that violates the slow-roll approximation, which implies amplification
of the curvature perturbation on small scales. Specifically, we consider
ultraslow-roll inflation as the intermediate period. At late time, inflation
should go back to the slow roll period so that it can end. This means that
there are two transitions of the second slow-roll parameter. In this paper, we
compare two different possibilities for the second transition: sharp and smooth
transitions. Focusing on effects generated by the relevant cubic
self-interaction of the curvature perturbation, we find that the bispectrum and
one-loop correction to the power spectrum due to the change of the second
slow-roll parameter vanish if and only if the Mukhanov-Sasaki equation for
perturbation satisfies a specific condition called Wands duality. We also find
in the case of sharp transition that, even though this duality is satisfied in
the ultraslow-roll and slow-roll phases, it is severely violated at the
transition so that the resultant one-loop correction is extremely large
inversely proportional to the duration of the transition.
