[1] Univ Granada, Dept Matemat Aplicada, E-18071 Granada, Spain
[2] Politecn Torino, Dept Math Sci, I-10100 Turin, Italy
[3] CNR, IMATI, I-27100 Pavia, Italy
[4] Natl Univ Cordoba, CIEM, CONICET, RA-5000 Cordoba, Argentina
[5] Natl Univ Cordoba, FaMAF, RA-5000 Cordoba, Argentina
[6] Univ Torin, Sch Management & Econ, I-10100 Turin, Italy