作者机构:[1]School of Mathematical Sciences, Huaqiao University, Quanzhou 362021, China, [2]School of Mathematical Sciences, Xiamen University, Xiamen 361005, China.
相关基金:Acknowledgments This work was partially supported by National Natural Science Foundation of China (Grant No. 11001090), the Fundamental Research Funds for the Central Universities (Grant No. 11QZR16).