1. Department of Mathematics, Gebze Technical University, 41400 Gebze, Kocaeli, Turkey
2. École Polytechnique de Montréal, C.P.6079 suc. Centre-ville, H3C 3A7 Montréal, Québec, Canada
3. Mathematical Institute, Faculty of Mathematics, Charles University, Sokolovská 83, 18675 Prague, Czech Republic