[Guan, Kaizhong] Univ S China, Dept Math, Hengyang 421001, Hunan, Peoples R China.
[Guan, Kaizhong] Hunan Normal Univ, Dept Math, Changsha 410081, Hunan, Peoples R China.
[Shen, Jianhua] Hangzhou Normal Univ, Dept Math, Hangzhou 310036, Zhejiang, Peoples R China.
[Shen, Jianhua] Coll Huaihua, Dept Math, Huaihua 418008, Hunan, Peoples R China.