For a length complex sequence , , thediscrete Fourier transform (DFT) is defined by
We are now in a position to have a full understanding of the transform kernel:
The kernel consists of samples of a complex sinusoid at discrete frequencies uniformly spaced between and the sampling rate. All that remains is to understand the purpose and function of the summation over of the pointwise product of times each complex sinusoid. We will learn that this can be interpreted as an inner product operation which computes the coefficient of projection of the signal onto the complex sinusoid . As such, , the DFT at frequency, is a measure of the amplitude and phase of the complex sinusoid at that frequency which is present in the input signal . This is the basic function of all transform summations (in discrete time) and integrals (in continuous time) and their kernels.