1. School of Mathematics and Computer Science; Fujian Normal University; Fuzhou Fujian 350108 China
2. Université Lille 1, LPP and INRIA Lille-Nord Europe; 59655 Villeneuve d'Ascq France
3. Université de Valenciennes; Valenciennes Cedex 09 59313 France
4. INRIA Paris-Rocquencourt; B.P. 105 Le Chesnay 78153 France