  Primordial black holes may arise through ultra slow-roll inflation. In this
work we study a toy model of ultra slow-roll inflation with a shallow dip. The
ultra slow-roll stage enhances the curvature perturbations and thus the
primordial scalar power spectrum. We analyze the features of the power spectrum
numerically and analytically, and then give a rough estimate of the lower and
upper bound of the enhancement. These large perturbations also produce second
order gravitational waves, which are in the scope of future observations.
