Amulti-mode solution is presented in the form of generalized double Fourier series with time-dependentcoefficients determined by the method of harmonic balance for undamped free vibrations.

• 给出了以广义双富里哀级数表示的无阻尼自由振动下的多模态解，其中与时间相关的系数由谐波平衡法确定。