1. Institute of Intelligent Computing Science, Shenzhen University, Shenzhen, China
2. School of Mathematics and Information Sciences, Yantai University, Yantai, China
3. Department of Mathematics, University of South Carolina, Columbia, S.C., U.S.A.
4. Department of Mathematics and Statistics, Missouri University of Science and Technology, Rolla, Mo., U.S.A.; and School of Mathematical Sciences, University of Electronic Science and Technology of China, Chengdu, China