傅里叶级数假设：周期函数\({f_k}(t)\)的形式为\({e^{ik{w_0}t}}\)，其中\({w_0}\)称为基频，\({f_k}(t)\)的频率为基频\({w_0}\)的整数倍，则有：
[\begin{array}{c}
f(t) = \sum\nolimits_{k = - \infty }^{ + \infty } {{c_k}{e^{ik{w_0}{\rm{t}}}}} \
= \cdots + {c_{ - 2}}{e^{ - 2i{w_0}t}} + {c_{ - 1}}{e^{ - 1i{w_0}t}} + {c_0}{e^{0i{w_0}t}} + {c_1}{e^{1i{w_0}t}} + {c_2}{e^{2i{w_0}t}} + \cdots
\end{array}]