作者机构:[1]School of Mathematics and Computer Science Ningxia University, Yinchuan 750021, P. R. China, [2]School of Mathematics and Computer Science Ningxia Normal University, Guyuan 756000, P. R. China
相关基金:We would like to thank the editor and referee for their very helpful comments and suggestions which improve this paper significantly. This research is supported by the National Natural Science Foundation of China (Nos. 11461053 and 11261043) (China), the School Foundation of Ningxia University (No. ZR1315) (China).