1. Department of Mathematics, Sichuan University, Chengdu, Sichuan 610064, China
2. School of Mathematics and Physics, Chongqing University of Posts and Telecommunications, Chongqing 400065, China
3. School of Mathematics, Statistics and Applied Mathematics, National University of Ireland, Galway, Ireland
4. Department of Mathematics, King Abdulaziz University, Jeddah, Saudi Arabia