Left panel: boundary of the set C_I0 (red) for the PSD (71). Parameters inside this area lead to PSD with a more than one inflection point. Unlike the power spectrum (4) S_rD(ω) can have a global maximum at ω ≈ Ω_s. C_I1 (middle) and C_Iq (right) show the internal structure of the set. The common feature of the three panels are the critical curves (shown by other colours), where the number of inflection points changes.