A A∩N 在最下面的函数是上 面四个函数的和 ·上面几个波形可以合 成下面的函数 FIGURE 4.1 The function at the bottom is the sum of the four functions above it. Fourier's idea in 1807 that periodic functions could be represented as a weighted sum of sines and cosines was met with skepticism.•在最下面的函数是上 面四个函数的和 •上面几个波形可以合 成下面的函数