When Does the Positive Semidefiniteness Constraint Help In Lifting   Procedures
  We study the lift-and-project procedures of Lov\'asz and Schrijver for 0-1 integer programming problems. We prove that the procedure using the positive semidefiniteness constraint is not better than the one without it, in the worst case. Various examples are considered. We also provide geometric conditions characterizing when the positive semidefiniteness constraint does not help. 
Levent Tuncel (University of Waterloo), Michel X. Goemans (MIT)