1. Research Center for Mathematics and Interdisciplinary SciencesShandong University266237 Qingdao P.R. China
2. Institute of MathematicsÉcole Polytechnique Fédérale de Lausanne (EPFL)Section 8 CH‐1015 Lausanne Switzerland
3. School of Information and MathematicsYangtze UniversityJingzhou 434023 P.R. China