$${E(t) = \sum_{n = -\infty}^{n = \infty} d_n \exp (i n \omega t) \Rightarrow E(t) = \frac{1}{2 \pi} \int_{- \infty} ^{\infty} E(\omega) \exp(i \omega t) d\omega \\ d_n = \frac{1}{T} \int_{-\frac{T}{2}}^{\frac{T}{2}} E(t) \exp(- n \omega t) dt \Rightarrow E(\omega) = \int_{-\infty}^\infty E(t) \exp(-i \omega t) dt}$$