@article{EmpiricalJACM2009,
        author = {K. Leyton-Brown and E. Nudelman and Y. Shoham },
        journal = {Journal of the {ACM}},
  title = {Empirical Hardness Models: Methodology and a Case Study on      Combinatorial Auctions },
        volume = "56"
        number=4,
	pages= {1-52}
	Year=2009
        }