Institution: | 1. Department of Mathematics, Qiongzhou University, Wuzhishan, Hainan, 572200, China;2. Department of Mathematics, Hainan Normal University, Haikou, Hainan, 571100, China;3. Department of Mathematics, West Virginia University, Morgantown, WV 26506-6310, United States;4. Ohio University Southern, Ironton, OH 45638, United States |