Institution: | a College of Mathematics and Information Science, Henan Normal University, Xinxiang, China b Department of Mathematics, Yeshiva University, 500 W. 185th St., New York, NY 10033, United States c Department of Applied Mathematics, Campus Box 526, University of Colorado at Boulder, Boulder, CO 80309, United States d Faculty of Mathematics, Magdeburg University, Universitätsplatz 2, D-39106 Magdeburg, Germany |