作者机构:[1]School of Mathematical Sciences, Dalian University of Technology and School of Sciences, Dalian Ocean University, Dalian 116025, China, [2]School of Mathematical Sciences, Dalian University of Technology, Dalian 116024, China
相关基金:Acknowledgments. This work is supported by the National Natural Science Foundation of China under projects Nos. 11071029, 11101064 and 91130007 and speciMized Research Fund for the Doctoral Program of Higher Education (20110041120039). We are grateful to the associate editor and anonymous referee's comments to improve the quality of the manuscript. The second author also appreciate the discussion with his student Miao Xiaonan.