Institution: | 1. Department of Mathematics, University of Salerno, Via Giovanni Paolo II, 132 Fisciano (Salerno), Italy;2. Mathematical Institute, Slovak Academy of Sciences, ?tefánikova 49, SK-814 73 Bratislava, Slovakia;3. Depart. Algebra Geom., Palacký Univer. 17. listopadu 12, CZ-771 46 Olomouc, Czech Republic |