作者机构:[1]Mathematics and Information College, Hebei Normal University, Hebei 050016, P. R. China, [2]School of Mathematical Sciences, Beijing Normal University, Beijing 100875, P. R. China
相关基金:Supported by the National Natural Science Foundation of China (Grant Nos. 10771051; 10971052), the National Natural Foundation of Hebei Province (Grant No. A2008000128), Educational Committee of Hebei Province (Grant No. 2009134) and Youth Science Foundation of Hebei Normal University (Grant No. L2008Q01)