[1] Chongqing Normal Univ, Sch Math Sci, Chongqing 401331, Peoples R China
[2] Indian Inst Technol BHU, Dept Math Sci, Varanasi 221005, Uttar Pradesh, India
[3] China Med Univ, Ctr Gen Educ, Taichung 40402, Taiwan
[4] Acad Romanian Scientists, Bucharest 50044, Romania