[1] Yanbian Univ, Coll Sci, Dept Math, 977 Gongyuan Rd, Jilin, Peoples R China
[2] Tokyo Univ Sci, Dept Math, 1-3 Kagurazaka, Tokyo, Tokyo 1628601, Japan
[3] Chiba Inst Technol, Dept Math, 2-1-1 Shibazono, Narashino, Chiba 2750023, Japan
[4] Osaka Metropolitan Univ, Grad Sch Sci, Dept Math, 3-3-138 Sugimoto, Sumiyoshi, Osaka 5588585, Japan