1. Department of Mathematics and Computer Science, University of Calabria, via Pietro Bucci 30B, 87036 Rende (CS), Italy. alviano@mat.unical.it
2. Department of Informatics, Bioengineering, Robotics and Systems Engineering, University of Genova, viale Francesco Causa 15, 16145 Genova (GE), Italy. dodaro@dibris.unige.it