Jianjun Liu 
Fudan University 
School of Mathematical Sciences 
200433 SHANGHAI 
P.R. CHINA 
E-mail: 071018018@fudan.edu.cn 
Xiaoping Yuan 
Fudan University 
School of Mathematical Sciences 
200433 SHANGHAI 
P.R. CHINA 
E-mail: xpyuan@fudan.edu.cn 
yuanxiaoping@hotmail.com