Institution: | 1. Department of Mathematical Analysis, Faculty of Science , Palacky University , 17. listopadu 12, 771 46 , Olomouc , Czech Republic andres@inf.upol.cz jan.andres@upol.cz;3. Centre PMF, Laboratory Marin Mersenne, Université Paris I Panthéon-Sorbonne , 90, Rue de Tolbiac, 75 634 , Paris Cedex 13 , France |