Department of Mathematics, Huaibei Normal University, Anhui 235000, P. R. China;
School of Mathematics and Computer Science, Nanjing Normal University,Jiangsu 210097, P. R. China;
School of Mathematics and Computer Science, Nanjing Normal University,Jiangsu 210097, P. R. China;
School of Mathematics and Computer Science, Nanjing Normal University,Jiangsu 210097, P. R. China;
Lianyungang Teacher's College, Jiangsu 222006, P. R. China;