“…Moreover, spectral analysis can be performed for nonlinear system through their NARMAX representation by computing the generalised frequency response functions (GFRFs) (Billings and Tsang, 1989a,b, Billings et al, 1990, Chua and Ng, 1979a,b, Jones, 2007, Lee and Chang, 2009, Zhang and Billings, 1993.…”