Equation (11.14) is called the complex Fourier series. Between the real and complex Fourier coefficients, the relationships
(11.16)
hold, where n > 0. from this we know the following relationship
(11.17)
Therefore, if these quantities are illustrated in the figure taking the order 
as the abscissa, the real part is symmetric about the ordinate axis, and the imaginary part is skew-symmetric about the origin. These complex Fourier coefficients can also be represented by
(11.18)