[1] Univ Seville, ETSI Informat, Dept Matemat Aplicada 1, Avda Reina Mercedes S-N, E-41012 Seville, Spain
[2] Univ Seville, ETSA, Dept Matemat Aplicada 1, Avda Reina Mercedes S-N, E-41012 Seville, Spain
[3] Univ Seville, Fac Matemat, Dept Algebra, C Tarfia S-N, E-41012 Seville, Spain