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Primordial black holes (PBHs) can be formed from the collapse oflarge-amplitude perturbation on small scales in the early universe. Such anenhanced spectrum can be realized by introducing a flat region in the potentialof single-field inflation, which makes the inflaton go into a temporaryultraslow-roll (USR) period. In this paper, we calculate the bispectrum ofcurvature perturbation in such a scenario. We explicitly confirm thatbispectrum satisfies Maldacena's theorem. At the end of the USR period, thebispectrum is generated by bulk interaction and field redefinition. At the endof inflation, bispectrum is generated only by bulk interaction. We alsocalculate the one-loop correction to the power spectrum from the bispectrum,called the source method. We find it consistent with the calculation ofone-loop correction from the second-order expansion of in-in perturbationtheory. In the last section of this paper, we write our response to criticismto our letter [arXiv:2211.03395] by [arXiv:2301.00599]. We argue that thecriticism is based on the incorrect use of Maldacena's theorem. After fixingsuch a mistake, we show that the one-loop correction in our letter isreproduced in the source method as well. This confirms our letter's conclusionthat rules out PBH formation from single-field inflation.

Response to criticism on "Ruling Out Primordial Black Hole Formation  From Single-Field Inflation'': A note on bispectrum and one-loop correction  in single-field inflation with primordial black hole formation