1. École Polytechnique de Montréal, C.P.6079 suc. Centre-ville, Montréal, Québec, Canada H3C 3A7
2. Department of Mathematics, FNSPE, Czech Technical University in Prague, Trojanova 13, Prague 120 00, Czech Republic
3. Mathematical Institute, Faculty of Mathematics, Charles University, Sokolovská 83, Prague 18675, Czech Republic