[1] Univ Firenze, Dipartimento Matemat & Informat Ulisse Dini, Viale Morgagni 67-A, I-50134 Florence, Italy
[2] Univ G dAnnunzio, Dipartimento Econ, Viale Pineta 4, I-65129 Pescara, Italy
[3] Univ Bucharest, Fac Math & Informat, Acad St 14, Bucharest 010014, Romania