1. School of Mathematics and Informatics, Henan Polytechnic University, Jiaozuo 454010, China
2. School of Mathematics and Physics, Hulunbuir University, Hulunbuir 021008, China
3. Independent Researcher, Dallas, TX 75252-8024, USA
4. Department of Mathematics, Texas A&M University-Kingsville, Kingsville, TX 78363-8202, USA