该函数用来计算任意函数的一阶偏导数(数值方法),用平面波展开法计算二维声子晶体带隙,基于互功率谱的时延估计。- This function is used to calculate the arbitrary function of the first order partial derivative (numerical methods), Computation Method D phononic bandgap plane wave, Based on the time delay estimation of power spectrum.