Returns the angular frequency vector corresponding to time vector t.
$$ w=\text{fftw}(t)=\frac{2\pi}{dt.M}[0,...,(M1)] $$
where M=length(t) .
fftw and all related functions have an option (option 1 below) that controls the format used to store the frequency domain data. When working with spectral data it is not possible to switch between formats; there are no functions to convert between formats. This implies that if you use option1=n to produce a spectrum with fft, then you must also use option1=n if you want to pass that same spectral data to invfft. Similarly, if you use option1=n for fft, then you also need to use option1=n with fftw to get the proper frequency vector corresponding to your spectrum. invfft and fftk work in the same way.
Syntax 
Description 

out = fftw(t); 
Returns the angular frequency vector corresponding to time vector t. 
fftw(t,option1,option2); 
option1
option2

Example
See fft or fftk examples.
See Also
List of commands , fft , fftk , invfft