1. Department of Mathematics, National Taiwan Normal University, Taipei 116, Taiwan.
2. Department of Mathematical Sciences, National Chengchi University, Taipei 116, Taiwan, and Department of Applied Mathematics, National University of Kaohsiung, Kaohsiung 811, Taiwan.
3. Nanjing Center for Applied Mathematics, Nanjing 211135, China, and Department of Applied Mathematics, National Yang Ming Chiao Tung University, Hsinchu 300, Taiwan.