Affiliation: | a Faculty of Mathematics and Computer Science, Adam Mickiewicz University, Umultowska 87, 61-614 Poznań, Poland b Institute of Mathematics of Electric Faculty, Poznah University of Technology, Piotrowo 3a, 60-965 Poznań, Poland c Institute of Mathematics, Szczecin University, Wielkopolska 15, 70-451 Szczecin, Poland |