[1] Southwest Univ, Sch Math & Stat, Chongqing 400715, Peoples R China
[2] Yeungnam Univ, Dept Math, Kyongsan 712749, South Korea
[3] Chongqing Univ Arts & Sci, Dept Math & Stat, Chongqing 402160, Peoples R China
[4] Univ Waterloo, Dept Pure Math, Waterloo, ON N2L 3G1, Canada