Yuanchu Hwang Soe-Tsyr Yuan UPS - An Ubiquitous Proximity eService for Trust Collaboration. 57-68 2007 conf/euc/2007w EUC Workshops http://dx.doi.org/10.1007/978-3-540-77090-9_6 db/conf/euc/eucw2007.html#HwangY07