Exact bounds on the ground-state energy of the infinite-U Hubbard model
  We give upper and lower bounds for the ground-state energy of the infinite-U Hubbard model. In two dimensions, using these bounds we are able to rule out the possibility of phase separation between the undoped-insulating state and an hole-rich state. 
Federico Becca, Luca Capriotti, Sandro Sorella, and Alberto Parola