[1] Yibin Univ, Dept Math, Yibin 644007, Sichuan, Peoples R China
[2] Gyeongsang Natl Univ, Dept Math Educ, Chinju 660701, South Korea
[3] Gyeongsang Natl Univ, RINS, Chinju 660701, South Korea
[4] Hong Kong Polytech Univ, Dept Appl Math, Kowloon, Hong Kong, Peoples R China