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 C n +1 2 and at least n peaks at wave-vectors k ij ≡( 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.