[1] Univ Gothenburg, Div Math, Dept Math Sci, S-41296 Gothenburg, Sweden
[2] Henan Polytech Univ, Sch Math & Informat Sci, Jiaozuo 454000, Peoples R China
[3] Beijing Normal Univ, Sch Math Sci, Minist Educ, Lab Math & Complex Syst, Beijing 100875, Peoples R China