Resonant multiple peaks in the induced gravitational waves

We identify analytically a multiple-peak structure in the energy-density spectrum of induced gravitational waves (GWs) generated at second-order from primordial scalar perturbations also with multiple (n) peaks at small scales k*i. The energy-density spectrum of induced GWs exhibits at most Cn+12 and at least n peaks at wave-vectors kij≡(k*i+k*j)/√3 due to resonant amplification, and, under the narrow-width approximation, it contains an universal factor that can be interpreted as a result of momentum conservation. We also extend these discussions to the case of non-Gaussian perturbations.