[1] Yibin Univ, Dept Math, Yibin 644007, Sichuan, Peoples R China
[2] Univ Queensland, Dept Math, Brisbane, Qld 4072, Australia
[3] Nanjing Univ Finance & Econ, Dept Appl Math, Nanjing 210003, Jiangsu, Peoples R China
[4] Nanjing Univ Finance & Econ, Sch Finance, Nanjing 210003, Jiangsu, Peoples R China