Institution: | 1. Department of Mathematics, Beifang University of Nationalities, Ningxia 750021, PR China;2. School of Mathematics, Capital Normal University, Beijing 100037, PR China;3. Department of Mathematics, Oklahoma State University, 401 Mathematical Sciences, Stillwater, OK 74078, USA;4. Department of Mathematics, College of Natural Science, Chung-Ang University, Seoul 156-756, South Korea |