Affiliations: Jan Chleboun, Faculty of Civil Engineering, Department of Mathematics, Thákurova 7, 166 29 Praha 6, Czech Republic, e-mail: jan.chleboun@fsv.cvut.cz; Michal Křížek, Institute of Mathematics of the Czech Academy of Sciences, Žitná 25, 115 67 Praha 1, Czech Republic, e-mail: krizek@math.cas.cz