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 kij equivalent to (k(*i) + k(*j)) / root 3 due to resonant ampli fi cation, 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.