1. University of Electronic Science and Technology of China,School of Mathematical Sciences,Chengdu,China,611731
2. University of Electronic Science and Technology of China,Yingcai Honors College,Chengdu,China,611731
3. Taiyuan Normal University,School of Mathematics and Statistics,Jinzhong,China,030619
4. National Research Council,Institute of Methodologies for Environmental Analysis,Tito Scalo,Italy,85050