[1] Shanghai Jiao Tong Univ, Dept Math, Shanghai 200240, Peoples R China
[2] Xinyang Normal Univ, Coll Math & Informat Sci, Xinyang 464000, Peoples R China
[3] Shandong Univ Sci & Technol, Coll Informat Sci & Engn, Qingdao 266510, Peoples R China
[4] Taishan Univ, Coll Math & Syst Sci, Tai An 271021, Shandong, Peoples R China