/956 傅里叶在1807年指出任意周期函数都可以表示为不同频率的正弦 /余弦和的形式。 N WWWM ∧AAAN 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.傅里叶在1807年指出任意周期函数都可以表示为不同频率的正弦 /余弦和的形式