[1] Wuhan Univ, Sch Math & Stat, Wuhan 430072, Peoples R China
[2] Wuhan Univ, Computat Sci Hubei Key Lab, Wuhan 430072, Peoples R China
[3] Univ British Columbia, Dept Math, Vancouver, BC V6T 1Z2, Canada
[4] Chinese Univ Hong Kong, Dept Math, Shatin, Hong Kong, Peoples R China