[1] Northwestern Polytech Univ, Dept Appl Math, Xian 710072, Shaanxi, Peoples R China
[2] Univ Wisconsin, Dept Math, 800 W Main St, Whitewater, WI 53190 USA
[3] Nankai Univ, Sch Math Sci, Tianjin, Peoples R China
[4] Nankai Univ, LPMC, Tianjin, Peoples R China
[5] Tsinghua Univ, Dept Math Sci, Beijing 100084, Peoples R China