作者机构:[1]Department of Mathematics, Lanzhou Jiaotong University, Lanzhou 730070, P. R. China, [2]School of Mathematics and Statistics, Lanzhou University, Lanzhou 730000, P. R. China
相关基金:Project supported by the National Natural Science Foundation of China (No.10571078) and the Natural Science Foundation of Gansu Province of China (No.3ZX062-B25-012)