[1] Princeton Univ, Dept Math, Fine Hall,Washington Rd, Washington, DC 08544 USA
[2] Oklahoma State Univ, Dept Math, Stillwater, OK 74078 USA
[3] Henan Polytech Univ, Sch Math & Informat Sci, Jiaozuo 454003, Henan, Peoples R China
[4] East China Univ Sci & Technol, Sch Sci, Shanghai 200237, Peoples R China