1. LAMFA, UMR 7352 CNRSUniversity of Picardie33 Rue Saint Leu 80039 Amiens France
2. School of Mathematics and StatisticsCentral China Normal University152 Luoyu road 430079 Wuhan China
3. Einstein Institute of MathematicsEdmond J. Safra CampusThe Hebrew University of JerusalemGivat Ram Jerusalem 9190401 Israel
4. Faculty of Mathematics and Computer ScienceJagiellonian University in KrakówŁojasiewicza 6 30‐348 Kraków Poland