A Tree Formation Algorithm for Data Aggregation Based on Estimate Cost in Sensor Networks YE Ning^1,2,WANG Ru-chuan^1,3 1.College of Computer,Nanjing University of Posts and Telecommunications,Nanjing,Jiangsu 210003,China; 2.Department of Information Science,Nanjing College for Population Programme Management,Nanjing,Jiangsu 210042,China; 3.State Key Laboratory for Novel Software Technology,Nanjing University,Nanjing,Jiangsu 210093,China)