1. School of Science , Dalian Jiaotong University , Dalian 116028, P. R. China
2. School of Mathematics and Statistics , Zhoukou Normal University , Zhoukou (Henan), P. R. China and Dipartimento di Matematica e Fisica , Università del Salento , Arnesano, P. O. Box 193, 73100 Lecce , Italy