1. Department of Mathematics, Kangwon National University, Chuncheon 24341, Gangwon-do, Republic of Korea
2. Department of Mathematics, Korea University, Seoul 02841, Republic of Korea
3. School of Computer Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China
4. Department of Mathematics, Zhengzhou University of Aeronautics, Zhengzhou 450046, China
5. Department of Mathematics, Harbin Institute of Technology at Weihai, Weihai 264209, China