Affiliation: | Mathematical Institute, Wroclaw University, pl. Grunwaldzki 2/4, 50-384 Wroclaw, Poland -- and -- Department of Mathematics and Information Technology, University of Warmia and Mazury, Zolnierska 14A, 10-561 Olsztyn, Poland ; Mathematical Institute, Wroclaw University, pl. Grunwaldzki 2/4, 50-384 Wroclaw, Poland -- and -- Institute of Mathematics of the Polish Academy of Sciences, Sniadeckich 8, 00-956 Warsaw, Poland |