1. Université Paris-Est, LAMA (UMR 8050), UPEMLV, UPEC, CNRS, 61 avenue du Général de Gaulle, 94010 Créteil Cedex 10, France. E-mail: bernard.ducomet@gmail.com
2. Institute of Mathematics of the Academy of Sciences of the Czech Republic, Žitná 25, 115 67 Praha 1, Czech Republic. E-mails: caggio@math.cas.cz, matus@math.cas.cz
3. Charles University, Faculty of Mathematics and Physics, Math. Institute of Charles University, Sokolovská 83, 186 75 Prague 8, Czech Republic. E-mail: pokorny@karlin.mff.cuni.cz