1. School of Mathematics and Statistics; Xidian University; Xi'an Shaanxi 710071 China
2. Guangxi Experiment Center of Information Science; Guilin University of Electronic Technology; Guilin Guangxi 541004 China
3. School of Science; Xi'an Technological University; Xi'an Shaanxi 710032 China
4. State Key Laboratory of Integrated Services Network; Xidian University; Xi'an Shaanxi 710071 China
5. Faculty of Mathematics; Kyushu University; Fukuoka 819-0395 Japan