[1] Univ Palermo, Dipartimento Matemat & Applicaz, I-90123 Palermo, Italy
[2] Ulyanovsk State Univ, Fac Math & Mech, Dept Algebra & Geometr Computat, Ulyanovsk, Russia
[3] Moscow MV Lomonosov State Univ, Fac Math & Mech, Dept Algebra, Moscow 119899, Russia