Elchanan Mossel Christos H. Papadimitriou Michael Schapira Yaron Singer VC v. VCG: Inapproximability of Combinatorial Auctions via Generalizations of the VC Dimension http://arxiv.org/abs/0905.1995 2009 CoRR abs/0905.1995 db/journals/corr/corr0905.html#abs-0905-1995 informal publication