1. College of Mathematics and Computer Science, Northwest Minzu University 1 , Lanzhou 730030, People’s Republic of China
2. Department of Mathematics, Incheon National University 2 , Incheon 22012, Republic of Korea
3. Faculty of Applied Mathematics, AGH University of Science and Technology 3 , 30-059 Kraków, Poland
4. Department of Mathematics, University of Craiova 4 , 200585 Craiova, Romania
5. China-Romania Research Center in Applied Mathematics, University of Craiova 5 , Craiova, Romania
6. College of Mathematical Sciences, Harbin Engineering University 6 , Harbin 150001, People’s Republic of China