1. Department of Mathematics and Statistics, Oakland University, Rochester, MI 48309, USA
2. Department of Computer Science, Brock University, St. Catharines, ON, Canada L2S 3A1, Canada
3. Department of Computer Science and Technology, Plymouth State University, Plymouth, NH 03264, USA
4. Department of Mathematics, Taiyuan University of Technology, Shanxi Taiyuan-030024, China