Jon Feldman S. Muthukrishnan Martin Pál Clifford Stein Budget optimization in search-based advertising auctions. 40-49 2007 conf/sigecom/2007 ACM Conference on Electronic Commerce http://doi.acm.org/10.1145/1250910.1250917 db/conf/sigecom/sigecom2007.html#FeldmanMPS07