The Fourier coefficients of the impulse train were used to obtain those
of a periodic rectangular wave in an easy way. 
Discrete Fourier series were introduced and formulae for their Fourier
coefficients obtained. These were computed for the disrete-time periodic
rectangular wave. The effect of an LTI system on a Fourier series
representation was described and frequency response defined. 
A discussion of desirable frequency responses for various kinds of filters
was begun.