1. School of Mathematical Sciences, Liaocheng University, Liaocheng, Shandong 252059, P. R. China
2. Department of Mathematics, Binzhou University, Binzhou, Shandong 256603, P. R. China
3. Department of Mathematics, Zhejiang Normal University, Jinhua, Zhejiang 321004, P. R. China
4. Department of Mathematics, Kunming University of Science and Technology, Kunming, Yunnan 650093, P. R. China
5. The Center for Economic Research, Shandong University, Ji'nan, Shandong 250100, P. R. China