1. Czech Technical University, Prague, Czech Republic
2. Technical University Braunschweig, Braunschweig, Germany.
3. Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic.
4. Department of Mathematics and Statistics, Masaryk University, Faculty of Sciences, Kotlářská 2, 611 37 Brno, Czech Republic.
5. Department of Mathematics and Statistics, York University, Toronto ON, M3J 1P3, Canada