1. College of Science, Minzu University of China, Beijing 100081, P. R. China
2. Faculty of Applied Mathematics, AGH University of Kraków, al. Mickiewicza 30, 30-059 Kraków, Poland
3. Faculty of Electrical Engineering and Communication, Brno University of Technology Technická 3058/10, Brno 61600, Czech Republic
4. Department of Mathematics, University of Craiova, Street A.I. Cuza No. 13, 200585 Craiova, Romania
5. Simion Stoilow Institute of Mathematics of the Romanian, Academy, Calea Griviţei No. 21, 010702, Bucharest, Romania
6. School of Mathematics, Zhejiang Normal University, Jinhua, Zhejiang 321004, P. R. China