1. Department of Mathematics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, P.R. China.
2. Centre de Physique Théorique (CPT), UMR-7332, Aix Marseille Université, Campus de Luminy, Case 907, 13288 Marseille cedex 9, France.
3. Department of Applied Mathematics, The Hong Kong Polytechnic University, Kowloon, Hong Kong, P.R. China.