1. School of Mathematics, Hangzhou Normal University, Hangzhou 311121, China
2. School of Mathematics and Informatics, Henan Polytechnic University, Jiaozuo 454010, China
3. Department of Mathematics, National Kaohsiung Normal University, Kaohsiung 82444, Taiwan
4. Institute of Mathematics, Henan Polytechnic University, Jiaozuo 454010, China
5. Independent Researcher, Dallas, TX 75252-8024, USA