Affiliation: | Faculty of Mathematics, University of Lódz, Banacha 22, 90-238 Lódz, Poland ; Faculty of Mathematics, University of Lódz, Banacha 22, 90-238 Lódz, Poland E. Gontarek ; Faculty of Mathematics, University of Lódz, Banacha 22, 90-238 Lódz, Poland |