“…The real dynamical behaviours of mechanical and structural systems are represented by nonlinear differential equations. For system with smooth nonlinearities (very often polynomial nonlinearities) perturbation methods have been developed to trace system responses, from them we can refer to Krilov-Bogoliubov-Mitropolski (KBM) [1,2,3,4,5,6,7,8,9], Harmonic Balance Method (HBM) [10,11,12], normal form [13,14,15,16,17,18,19,20,21,22] and multiple scale methods [6,23,24,25,26,27]. Meanwhile, many other methods exists which are omitted here [23].…”