title: Is there any practical difference between the FFT and the inverse FFT?
body: <p>FFT libraries such as FFTW or numpy.fft typically provide two functions <code>fft()</code> and <code>ifft()</code> (and special versions thereof for real valued input). Those functions appear to be defined such that </p>
  
  <pre><code>ifft(fft(X)) == X
  </code></pre>
  
  <p>and</p>
  
  <pre><code>fft(X) == constant_factor * reverse(ifft(X))
  </code></pre>
  
  <p>Are there any practical or technical reasons to consider when choosing between <code>ifft()</code> and <code>fft()</code> operating on complex input? While the interpretations of <code>X</code> (signal, time or space domain) and <code>fft(X)</code> (spectrum, frequency domain) are different, does it ever matter computationally?</p>