text: Theoretical analysis of these results21,22,25,28,30 added 
  one more degree of complexity to the issue. It is tempt- 
  ing to analyze the temperature dependence of WK and 
  relate it to the observed behavior of the optical integral, 
  and some earlier works25,28,30 followed this route. In the 
  experiments, however, optical conductivity is integrated 
  only up to a certain frequency ωc, and the quantity which 
  is actually measured is 
  
  In a conventional BCS superconductor the only rele- 
  vant scales are the superconducting gap ∆ and the impu- 
  rity scattering rate Γ. Both are generally much smaller 
  than the Fermi energy, so the optical integral should be 
  almost T -independent, i.e., the spectral weight lost in a 
  superconducting state at low frequencies because of gap 
  opening is completely recovered by the zero-frequency δ- 
  function. 
  In a clean limit, the weight which goes into 
  a δ−function is recovered within frequencies up to 4∆. 
  This is the essence of FGT sum rule 2,3. In a dirty limit, 
  this scale is larger, O(Γ), but still WK is T -independent 
  and there was no “violation of sum rule”. 