[1] Southern Univ Sci & Technol, Dept Math, Shenzhen 518055, Guangdong, Peoples R China
[2] Peking Univ, Sch Math Sci, Beijing 100871, Peoples R China
[3] Harbin Univ Sci & Technol, Rongcheng Campus, Harbin 150080, Heilongjiang, Peoples R China
[4] Univ Melbourne, Sch Math & Stat, Parkville, Vic 3010, Australia