1. College of Science, East China Jiaotong University, Nanchang 330013, China
2. Faculty of Applied Mathematics, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland
3. Institute of Mathematics, Physics and Mechanics, 1000 Ljubljana, Slovenia
4. Institute of Mathematics “Simion Stoilow” of the Romanian Academy, P.O. Box 1-764, Bucharest 014700, Romania
5. Department of Mathematics, Heilongjiang Institute of Technology, Harbin 150050, China