2. Predicting the subharmonic and superharmonic frequencies
In a linear system excited by a harmonic excitation with a given frequency, the system will respond at the very same frequency with a certain magnitude and a certain phase angle relative to the input. This is the linear frequency response theory. In a nonlinear system that is also excited by a given harmonic excitation, only the excitation frequency cannot reveal the complexity of the response. Besides the driving frequency, we also need to consider the subharmonic and superharmonic frequencies. These frequencies are induced by the nonlinear terms in the system equation. This is the nonlinear frequency response theory. The subharmonic and superharmonic frequencies in the response of Eq. (1) can be predicted by a simple analysis that follows.