For the DTFT, we proved in Chapter 2 (p.
p. ) the stretch theorem (repeat theorem) which
relates upsampling (``stretch'') to spectral copies (``images'') in
the DTFT context; this is the discrete-time counterpart of the scaling
theorem for continuous-time Fourier transforms
(§B.4). Also, §2.3.12 discusses the downsampling
theorem (aliasing theorem) for DTFTs which relates downsampling to
aliasing for discrete-time signals. In this section, we review the
main results.