[1] South China Normal Univ, Sch Math Sci, Guangzhou 510631, Guangdong, Peoples R China
[2] South China Univ Technol, Sch Math, Guangzhou 510641, Guangdong, Peoples R China
[3] Champlain Coll St Lambert, Dept Math, Quebec City, PQ J4P 3P2, Canada
[4] McGill Univ, Dept Math & Stat, Montreal, PQ H3A 2K6, Canada