[1] Texas A&M Int Univ, Dept Math & Phys, Laredo, TX 78041 USA
[2] Guangzhou Univ, Inst Comp Sci & Technol, Guangzhou 510006, Guangdong, Peoples R China
[3] Anhui Jianzhu Univ, Sch Math & Phys, Hefei 230601, Anhui, Peoples R China
[4] Univ Mississippi, Dept Math, University, MS 38677 USA