Authors
Jacek Hejduk
Faculty of Mathematics and Computer Science, University of Łódź, Łódź, Poland
Gertruda Ivanova
Institute of Exact Science and Technology, Pomeranian University in Słupsk, Słupsk, Poland
Renata Wiertelak
renata.wiertelak@wmii.uni.lodz.pl
Faculty of Mathematics and Computer Science, University of Łódź, Łódź, Poland