1. Postdoctoral Research Station of Mathematics, School of Mathematical Sciences Hebei Normal University Shijiazhuang 050024 China
2. Department of Automation, Key Laboratory of System Control and Information Processing, Ministry of Education of China Shanghai Jiao Tong University Shanghai 200240 China
3. Department of Mathematics, Concordia College Moorhead 56562 MN USA
4. School of Mathematics Sun Yat‐Sen University Guangzhou 510275 China