We now consider discrete functions, as opposed to the continuous functions of the previous lectures. Discrete functions are defined at discrete positions:
f(x) is defined on integer values of x between 0 and N−1.
The DFT is the discrete analogue of the continuous FT, with a sum replacing the integral:
F(u)=N−1∑x=0f(x)e−i2πxNu
The 2πxN term varies in [0,2π) as x varies in [0,N).
When u=1, the sinusoid corresponding to ei2πxNu has a single cycle in [0,N). When u=2, the sinusoid has two cycles in [0,N).
So u corresponds to the wavenumber in the continuous FT. That is, u is the number of cycles in the range [0,N).
The inverse DFT is also analagous to its continuous counterpart:
f(x)=1NN−1∑u=0F(u)ei2πuNx