1 Constructing a Message-Pruning Tree with Minimum Cost for Tracking Moving Objects in Wireless Sensor Networks is NP-Complete and an Enhanced Data Aggregation Structure Bing-Hong Liu, Wei-Chieh Ke, Chin-Hsien Tsai, and Ming- Jer Tsai, Member, IEEE The authors are with the Department of Computer Science, National Tsing Hua University, HsingChu, Taiwan 30013, ROC. E-mail: lbh@cs.nthu.edu.tw, u882522@alumni.nthu.edu.tw, satay@dclab.cs.nthu.edu.tw, mjtsai@cs.nthu.edu.tw. July 30, 2007 DRAFT