1. Institute of Mathematics; Polish Academy of Sciences; ul. Kopernika 18 Wroclaw 51-617 Poland
2. Institute of Mathematics and Computer Science; Wrocław University of Technology; ul. Wybrże Wyspiań-skiego 27 Wrocław 50-370 Poland
3. Laboratoire de Mathématiques; Université Clermont-Ferrand II; Campus des Cézeaux, 24 av. des Landais Aubière cedex 63177 France