[Xu, Youjun] Fudan Univ, Sch Math Sci, Shanghai 200433, Peoples R China.^[Xu, Youjun;Ouyang, Zigen] Univ South China, Sch Math & Phys, Hengyang 421001, Peoples R China.^[Xiao, Cuie] Hunan City Univ, Dept Math & Comp Sci, Yiyang 413000, Peoples R China.