1. School of Mathematics and Information Science, Weifang University, Weifang, Shandong 261061, China
2. School of Mathematical and Informational Sciences, Yantai University, Yantai, Shandong 264005, China
3. Department of Mathematics and Statistics, Curtin University of Technology, Perth, WA 6845, Australia
4. School of Mathematical Sciences, Qufu Normal University, Qufu 273165, Shandong, China