[1] Univ New England, Sch Sci & Technol, Dept Math, Armidale, NSW 2351, Australia
[2] Qufu Normal Univ, Dept Math, Qufu 273165, Shandong, Peoples R China
[3] China Three Gorges Univ, Coll Sci, Dept Math, Yichang City 443002, Hubei Province, Peoples R China
[4] Southeast Univ, Dept Math, Nanjing 210018, Peoples R China
[5] Xuzhou Normal Univ, Dept Math, Xuzhou 221116, Peoples R China