On the excluded space in applications of Feshbach projection formalism
  Various model applications in nuclear structure and reactions have been formulated starting with the Feshbach projection formalism. In recent studies a truncated excluded space has been enumerated to facilitate calculation and identify a convergence in expansions within that truncation. However, the effect of any remainder must be addressed before results from such can be considered physical. 
S. Karataglidis and K. Amos