1. Department of Applied Mathematics, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan
2. Department of Mathematical Sciences, Loughborough University, Loughborough, Leicestershire LE11 3TU, United Kingdom
3. Institut für Theoretische Physik, Westfälische Wilhelms-Universität Münster, Wilhelm Klemm Strasse 9, D-48149 Münster, Germany
4. Center of Nonlinear Science (CeNoS), Westfälische Wilhelms Universität Münster, Corrensstr. 2, 48149 Münster, Germany