A study on the coexistence of BEC and BCS states
  We pointed out in this work that in dealing with the BEC-BCS crossover problem, the dynamic effect of <b(t)>is not negligible. Accordingly, an equation of motion approach was devised to calculate the Green$^{\prime}$s functions. Based on our result, we concluded that instead of crossover, BCS states and Bose-Einstein condensation always coexist. 
C. D. Hu