1. School of Mathematics and Statistics , HNP-LAMA , Central South University , Changsha 410083, Hunan , P. R. China ; and Department of Mathematics, University of Craiova, 200585 Craiova, Romania
2. Faculty of Applied Mathematics , AGH University of Science and Technology , al. Mickiewicza 30, 30-059 Kraków , Poland ; and Brno University of Technology, Faculty of Electrical Engineering and Communication, Technická 3058/10, Brno 61600, Czech Republic; and Department of Mathematics, University of Craiova, 200585 Craiova, Romania; and Simion Stoilow Institute of Mathematics of the Romanian Academy, 010702 Bucharest, Romania; and School of Mathematics, Zhejiang Normal University, Jinhua, Zhejiang, 321004,
3. School of Mathematics and Statistics , HNP-LAMA , Central South University , Changsha 410083, Hunan , P. R. China
4. Department of Mathematics , Suzhou University of Science and Technology , Suzhou 215009, Jiangsu , P. R. China