Low Density Limit of BCS Theory and Bose-Einstein Condensation of   Fermion Pairs
  We consider the low density limit of a Fermi gas in the BCS approximation. We show that if the interaction potential allows for a two-particle bound state, the system at zero temperature is well approximated by the Gross-Pitaevskii functional, describing a Bose-Einstein condensate of fermion pairs. 
Christian Hainzl, Robert Seiringer
math-ph cond-mat.quant-gas math.MP