Institute of Mathematics and Statistics Masaryk University Kotl´aˇrsk´a 2 CZ–611-37 Brno CZECH REPUBLIC
Mathematical Institute Slovak Academy of Sciences ˇStef´anikova 49 SK–814-73 Bratislava SLOVAKIA
Faculty of Natural Sciences Matej Bel University Tajovsk´eho 40 SK–974-01 Bansk´a Bystrica SLOVAKIA