[1] Changsha Univ Sci & Technol, Coll Math & Comp Sci, Changsha 410114, Hunan, Peoples R China
[2] Guangzhou Univ, Sch Math & Informat Sci, Guangzhou 510006, Guangdong, Peoples R China
[3] Chongqing Normal Univ, Coll Math, Chongqing 400047, Peoples R China
[4] Wilfrid Laurier Univ, Dept Math, Waterloo, ON N2L 3C5, Canada