[1] Univ Ljubljana, Fac Educ, Ljubljana, Slovenia
[2] Univ Ljubljana, Fac Math & Phys, Ljubljana, Slovenia
[3] Inst Math Phys & Mech, Ljubljana, Slovenia
[4] Budapest Univ Technol & Econ, Dept Differential Equat, Budapest, Hungary
[5] Eotvos Lorand Univ, Dept Appl Anal & Computat Math, Budapest, Hungary
[6] Budapest Univ Technol & Econ, Dept Anal, Budapest, Hungary
[7] MTA ELTE NumNet Res Grp, Budapest, Hungary