1. Department of Mathematics, College of Charleston, Charleston, South Carolina, USA;
2. Department of Mathematics, Zhengzhou University Zhengzhou, Henan, P. R. China;
3. Department of Mathematical Sciences, United States Military Academy, West Point, New York, USA;
4. Department of Mathematics, Wilfrid Laurier University, Waterloo, Ontario, Canada;
5. College of Mathematics and Information Science, Hebei Normal (Teachers) University, Shijiazhuang, Hebei, P. R. China