1. Faculty of Mathematics and Computer Sciences Łódź University ul . Stefana Banacha 22 90-238 Łódź Poland
2. Institute of Mathematics Pomeranian University in Słupsk Kozietulskiego 6–7 76-200 Słupsk Poland
3. Institute of Mathematics Łódź Technical University ul . Wólczańska 215 93–005 Łódź Poland