[1] Univ Ferrara, Dept Math & Comp Sci, I-44121 Ferrara, Italy
[2] Univ Bourgogne Franche Comte, CNRS UMR 6623, Lab Math Besancon, 16 Route Gray, F-25030 Besancon, France
[3] Uniwersytet Marii Curie Sklodowskiej, Plac Marii Curie Sklodowskiej 1, PL-20031 Lublin, Poland