We study the production of gravitational waves from primordial scalar perturbations in doubleinflection-point inflation, in which one of the inflection points predicts the power spectra consistent with CMB observations at large scales and the other generates a large peak in the power spectrum of scalar perturbations at small scales. We calculate the energy spectrum of the reduced gravitational waves and find that the gravitational-wave signal can be detected by future space-based laser interferometers. * xwutao@itp.ac.cn

Recently it was shown that cosmic anisotropy can be well tested using either standard siren measurement of luminosity distance d L (z) from gravitational-wave (GW) observation or dispersion measure (DM(z)) from fast radio burst (FRB). It was also observed that the combined measurement of d L (z) · DM(z) from the GW/FRB association system as suggested in some of FRB models is more effective to constrain cosmological parameters than d L (z) or DM(z) separately. In this paper, we show that this upgraded siren from the combined GW/FRB observations could test cosmic anisotropy with a double relative sensitivity compared to the usual standard siren from GW observations only.