作者机构:[1]Department of Mathematics, Harbin Institute of Technology, Harbin 150001, China., [2]Department of Mathematics, Zhejiang University, Hangzhou 310027, China.
相关基金:Supported by Natural Science Foundations of China (11101108, 11171301, 10771191 and 10471124) and Natural Science Foundation of Zhejiang Province of China (Y6090105). Acknowledgments. The authors wish to express their thanks to the referees for their valuable comments and suggestions.