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