Sudhir Kumar Singh Vwani P. Roychowdhury Himawan Gunadhi Behnam Attaran Rezaei Capacity constraints and the inevitability of mediators in adword auctions http://arxiv.org/abs/0709.0204 2007 CoRR abs/0709.0204 db/journals/corr/corr0709.html#abs-0709-0204 informal publication