$$W(N)={\mathrm{A.}}_0-{\mathrm{A.}}_{1.}\mathrm{余弦}\left(\frac{2.\pi N}{L-1.}\right)+{\mathrm{A.}}_{2.}\mathrm{余弦}\left(\frac{4.\pi N}{L-1.}\right)-{\mathrm{A.}}_{3.}\mathrm{余弦}\left(\frac{6.\pi N}{L-1.}\right)+{\mathrm{A.}}_{4.}\mathrm{余弦}\left(\frac{8.\pi N}{L-1.}\right),$$