作者机构:[1]College of Mathematics and Information Science, Hebei Normal University, Hebei 050016, China, [2]7227 Mail Box, Beijing 100072, China
相关基金:Foundation item: the National Natural Science Foundation of China (No. 10571043; 10671053); the Natural Science Foundation of Hebei Province (No. 102132) and the Fundation of the Education Department of Hebei Province (No. 2004108).