[1] China Univ Min & Technol, Sch Math, Xuzhou 221116, Jiangsu, Peoples R China
[2] China Univ Min & Technol, Inst Math Phys, Xuzhou 221116, Jiangsu, Peoples R China
[3] Shandong Univ Sci & Technol, Coll Math & Syst Sci, Qingdao 266590, Shandong, Peoples R China
[4] Harbin Inst Technol, Dept Math, Harbin 150001, Heilongjiang, Peoples R China