Affiliation: | a Department of Mathematics, National Changhua University of Education, Changhua 500, Taiwan, ROC b Department of Mathematics, The Chinese University of Hong Kong Shatin, Hong Kong c Department of Mathematics, University of Connecticut, Storrs, CT 06269-3009, USA d Department of Applied Mathematics, National Sun Yat-sen University, Kaohsiung 804, Taiwan, ROC e National Center for Theoretical Sciences, Taiwan, ROC |