作者机构:[1]College of Sciences, China University of Mining and Technology, Xuzhou 221116, China, [2]Department of Applied Mathematics, University of Western Ontario,London, ON N6A 5B7, Canada
相关基金:The authors would like to thank the anonymous referees for their careful reading and helpful suggestions. This work was supported in part by the National Natural Science Foundation of China (Grant No. 11101422), the Fundamental Research Funds for the Central Universities (Grant No. 2012QNA36), and Qing Lan Project.