作者机构:[1]Department of Mathematics, Tianjin Polytechnic University, Tianjin 300160, P. R. China, [2]College of Applied Sciences, Beijing University of Technology, Beijing 100124, P. R. China
相关基金:Supported by the National Natural Science Foundation of China (Grant No. 10871012) and the Natural Science Foundation of Beijing (Grant No. 1082003).