title: FFT coefficients question
body: <p>I'm a software engineer working on DSP for the first time.</p>
  
  <p>I'm successfully using an FFT library that produces frequency spectrums. I also understand how the FFT works in terms of its inputs and outputs, in particular the contents of the two output arrays:
  <img src="https://i.stack.imgur.com/JDdSG.png" alt="enter image description here"></p>
  
  <p>Now, my problem is that I'm reading some new research reports that suggest that I extract: <strong>"the energy, variance, and sum of FFT coefficients"</strong>.</p>
  
  <ol>
  <li><p>What are the 'FFT coefficients'? Are those the values of the Real and Imaginary arrays shown above, which (from my understanding) correspond to the amplitudes of the constituent cosine and sine waves?</p></li>
  <li><p>What is the 'energy' of the FFT coefficients? Is that terminology from statistics or from DSP?</p></li>
  </ol>