We continued with Chapter 4.
We saw how the various rules for how modification of signals affect their Fourier transforms
can be used to speed up calculation of Fourier transforms.
We used duality to compute the Fourier transform of 2/(1+t^2), using that 2/(1+w^2) is the
transform of a familiar signal. 
We saw the advantage of working in the frequency domain for certain questions such as using
Parseval to compute the energy of a signal.
In particular, for an LTI system with frequency response H(jw), the F.T. of the output is
just H(jw) times the F.T. of the input.
Discussed lowpass filters and designing their H(jw). Discussed frequency-selective filters.
We finished an example finding the impulse response of a stable, causal LTI
system given by a differential equation, by working in the frequency domain,
using partial fractions. A similar technique then found the output for a
given input.