text: One particular case, studied in detail for conventional 
  superconductors, 
  is the redistribution of the spectral 
  weight between normal and superconducting states. This 
  is known as Ferrel-Glover-Tinkham (FGT) sum rule:2,3 
  
  πnse2 
  2m 
  
  ∞ 
  Re σN S(Ω) =  (2) 
  0+  0+ 
  Z  Z 
  
  In reality, as already pointed out by Hirsch, there is no 
  true violation as the change of the total spectral weight  where ns is the superﬂuid density, and πnse2/(2m) is 