fourier transform derivation

[beri]

hi everybody ,
i'd like to ask a question about derivation of fourier transform . when we derive fourier transform from fourier series why we use envelope function (T ak) what is the exact reason of this and what is envelope exactly
thanks

 

[andre_luis]

what is envelope exactly

The envelope function is a model of the signal that you are properly working, let´s say, the subject of interest. In time domain would represent the shape bounds of the amplitude of that signal, but in frequency domain I´m not sure if there exists the same graphical meaning.