1. Department of Technical Mathematics; Czech Technical University; Karlovo nám. 13, 121 35 Prague 2 Czech Republic
2. Mathematical Institute of the Academy of Sciences of the Czech Republic; Žitná 25, 115 67 Praha 1 Czech Republic
3. Université du Sud, Toulon-Var, Mathématiques; B.P. 20132 83957 La Garde Cedex France
4. Department of Mathematics; Darmstadt University of Technology; 64289 Darmstadt Germany