1. Department of Mathematics and Computer Science, University of Perugia, Italy. marco.baioletti@unipg.it
2. Department of Mathematics and Computer Science, University of Florence, Italy. gabriele.dibari@unifi.it
3. Department of Mathematics and Computer Science, University of Perugia, Italy. alfredo.milani@unipg.it
4. Department of Humanities and Social Sciences, University for Foreigners of Perugia, Italy. valentino.santucci@unistrapg.it