1. Department of Mathematical Sciences, Tsinghua University, Beijing 100084, P. R. China
2. UFR de Mathématiques, Laboratoire Paul Painlevé, UMR 8524, Université de Lille 1, 59655 Villeneuve d'Ascq, France
3. Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong SAR, P. R. China