1. Department of Applied MathematicsNanjing Forestry UniversityNanjing 210037 P.R. China
2. Department of MathematicsCollege of ScienceKing Saud UniversityP.O. Box 2455 Riyadh 11451 Saudi Arabia
3. School of Mathematics (Zhuhai)Sun Yat‐sen UniversityZhuhai Guangdong 519082 P.R. China