1. School of Mathematics and Computational ScienceHunan University of Science and TechnologyXiangtan Hunan China
2. Department of MathematicsCity University of Hong KongKowloon Hong Kong
3. School of Science and EngineeringChinese University of Hong KongShenzhen Guangdong China
4. Department of MathematicsUniversity of Louisiana at LafayetteLafayette LA USA