1. Department of Mathematics and Computer Science, Tongling University, Tongling 244000, China
2. School of Mathematical Sciences and Institute of Finance and Statistics, Nanjing Normal University, Nanjing 210023, China
3. Department of Mathematics, University of Bielefeld, 33615 Bielefeld, Germany
4. School of Mathematics and Information Technology, Nanjing Xiaozhuang University, Nanjing, Jiangsu 211171, China