Finished echo example (2.64).
Did questions similar to the HW.
We did an introduction to Fourier series.
We saw how a continuous-time LTI system transforms input
x(t) = e^(st) into an output y(t) = H(s) x(t), and how a discrete-time
system transforms an input x[n] = z^n into an output H(z) x[n].
Considered periodic continuous-time signals x(t+T) = x(t) for all t.
Examples include phi_k(t) = e^(jk w_0 t), k any integer and w_0 = 2\pi /T, 
and any superposition x(t) = \sum a_k phi_k(t) of these.
This is called the Fourier series representation of x(t).