1. Department of Mathematics, School of Science, Penn State Erie, The Behrend College, Penn State University, PA 16563, USA
2. Department of Mathematics, College of William and Mary, Williamsburg, VA 23187-8795, USA
3. School of Mathematics, Harbin Normal University, Harbin, Heilongjiang 150080, China
4. Department of Mathematics and Statistics, Mississippi State University, Mississippi State, MS 39762, USA