text: The result most relevant to the experiments on the 
  cuprates is obtained for the spin ﬂuctuation model. 
  We found that at strong coupling, the zero-crossing of 
  δW (ωc) occurs at a frequency which increases with the 
  coupling strength and may become larger than the band- 
  width at a truly strong coupling. Still, at even larger 
  frequencies, ∆W (ωc) is negative. 