title: Take fft and ifft for a few specific frequencies
body: <p>I have a question regarding to the <code>fft</code> and <code>ifft</code> functions.</p>
  
  <p>Suppose <code>A</code> is an array that includes 365 days of data:</p>
  
  <pre><code>fft_output = scipy.fftpack.fft(A)
  </code></pre>
  
  <p>It gives me the coefficients related to all frequencies. I only need three frequencies <code>(0, 1/365, 1/182)</code>. So I think <code>fft_output[0]</code> and <code>fft_output[364]</code>, <code>fft_output[181]</code> would be correct option for my desired frequencies. </p>
  
  <p>Now I would like to take <code>ifft</code> just by using these three frequencies for 365 days. I didn't figure out how can I do this by using the <code>ifft</code> function. </p>
  
  <p>I would appreciate if anyone guides me.</p>
  
  <p>Thank you in advance</p>