[1] Univ Politecn Cataluna, Barcelona Tech, CSIC, Dept Matemat,IRI, Ave Diagonal 647, E-08028 Barcelona, Spain
[2] Univ Politecn Cataluna, Barcelona Tech, CSIC, Inst Robot & Informat Ind,IRI, Ave Diagonal 647, E-08028 Barcelona, Spain
[3] Univ Politecn Cataluna, Barcelona Tech, Dept Matemat, Ave Diagonal 647, E-08028 Barcelona, Spain
[4] Martin Luther Univ Halle Wittenberg, Inst Math, Theodor Lieser Str 5, D-06099 Saale, Sachsen Anhalt, Germany
[5] Leibniz Univ Hannover, Inst Algebra Geometrie, Welfengarten 1, D-30161 Hannover, Niedersachsen, Germany