作者机构:[1]*Department of MathematicsHarbin Institute of Technology, Harbin Heilongjiang 150001, P. R. China, [2]School of ScienceChina University of Petroleum (East China) Qingdao 266580, P. R. China
相关基金:Acknowledgments The authors would like to thank the reviewers' constructive suggestions which have improved the presentation of the paper. This research is supported by National Natural Science Foundation of China (No. 11371111), the Research Fund for the Doctoral Program of Higher Education of China (No. 20122302110044) and Shandong Provincial Natural Science Foundation, China (No. ZR2013AQ023).