1. College of Mathematics and Systems Science, Shandong University of Science and Technology, Qingdao 266590, P. R. China
2. Dipartimento di Scienze Pure e Applicate (DiSPeA), Università degli Studi di Urbino Carlo Bo, Urbino, 61029, Italy
3. School of Mathematics, Zhejiang Normal University, Jinhua, 321004, P.R. China