[1] Univ Seville, Dept Matemat Aplicada 1, Seville, Spain
[2] Charles Univ Prague, Dept Appl Math, CR-11800 Prague, Czech Republic
[3] Charles Univ Prague, Inst Theoret Comp Sci, CR-11800 Prague, Czech Republic
[4] Univ Politecn Cataluna, Dept Matemat Aplicada 4, Barcelona, Spain