作者机构:[1]College of Math. and Inform. Sci., Hebei Normal Univ., Shijiazhuang 050016, China, [2]Dept. of Math., Shijiazhuang Univ., Shijiazhuang 050035, China
相关基金:Supported by the National Natural Science Foundation of China (10701032); Natural Science Foundation of Hebei Province (A2008000132); the Doctoral Foundation of Hebei Normal University (L2005B02)