[1] Jilin Univ, Sch Math, Changchun 130012, Jilin, Peoples R China
[2] Changchun Univ Technol, Sch Basic Sci, Changchun 130012, Jilin, Peoples R China
[3] Ernst Moritz Arndt Univ Greifswald, Inst Math & Informat, D-17487 Greifswald, Germany
[4] Shenzhen Univ, Coll Math & Computat Sci, Shenzhen 518060, Peoples R China