A key property of sinusoids is that they are orthogonal at different frequencies. That is,
This is true whether they are complex or real, and whatever amplitude and phase they may have. All that matters is that the frequencies be different. Note, however, that the durations must be infinity (in general).
For length
sampled sinusoidal signal segments, such as used
by the DFT, exact orthogonality holds only for the harmonics of
the sampling-rate-divided-by-
, i.e., only for the frequencies (in Hz)
These are the only frequencies that have a whole number of periods in
The complex sinusoids corresponding to the frequencies
are
These sinusoids are generated by the