作者机构:[1]School of Mathematics and Statistics, Huazhong University of Science and Technology, Wuhan 430074, China
相关基金:Acknowledgments. The authors are grateful to the referees for carefully reading the preliminary version of the manuscript. Their valuable suggestions largely improve the quality of this paper. The research is supported by the National Nature Science Foundation of China (No.10871078), 863 Program of China (No. 2009AA044501) and Postgraduate Innovation Fund of Huazhong University of Science and Technology (No. HF-08-02-2011-011).