[1] Univ Bologna, Dipartimento Elettron Informat & Sistemist, I-40136 Bologna, Italy
[2] Univ Roma La Sapienza, Dipartimento Informat & Sistemist, I-00184 Rome, Italy
[3] Washington Univ, Dept Syst Sci & Math, St Louis, MO 63130 USA
[4] Univ Ancona, Dipartimento Elettron & Automat, I-60131 Ancona, Italy