We are going to be performing manipulations on signals and their Fourier Transform throughout this class. It is important to understand how changes we make in one domain affect the other domain. The Fourier theorems are helpful for this purpose.
Derivations of the Fourier theorems for the DTFT case may be found in Chapter 2 of the text, and in Mathematics of the DFT1(Music 320 text) for the DFT case.