Affiliations: Miroslav Rozložník, Institute of Mathematics of the Czech Academy of Sciences, Žitná 25, 115 67 Praha 1, Czech Republic, e-mail: miro@math.cas.cz; Stanislav Sysala, Institute of Geonics of the Czech Academy of Sciences, Studentská 1768, 708 00 Ostrava, Czech Republic, e-mail: stanislav.sysala@ugn.cas.cz