1. Dipartimento di Matematica; Università degli Studi di Salerno; Via Giovanni Paolo II 132 - 84084 Fisciano Italia
2. Institut für Mathematik; Universität Würzburg; Emil-Fischer-Strasse 30 97074 Würzburg Germany
3. Hugo Steinhaus Center, Institute of Mathematics and Computer Science; Wroclaw University of Technology; Wyspia'nskiego 27 50-370 Wroclaw Poland