Ivan Osipkov Eugene Y. Vasserman Nicholas Hopper Yongdae Kim Combating Double-Spending Using Cooperative P2P Systems. 41 2007 conf/icdcs/2007 ICDCS http://doi.ieeecomputersociety.org/10.1109/ICDCS.2007.91 db/conf/icdcs/icdcs2007.html#OsipkovVHK07