1. Faculty of Mathematics and Computer Science, A. Mickiewicz University, Umultowska 87, 61-614 Poznań, Poland
2. Institute of Mathematics, Poznań University of Technology, Piotrowo 3A, 60-965 Poznań, Poland
3. Institute of Mathematics, Lodz University of Technology, Ul. Wólczańska 215, 90-924 Łódź, Poland