1. Engineering and Computer Sciences, Nebrija University, Pirineos 55, Madrid, E-28040, Spain
2. Faculty of Mathematics, Complutense University, Plaza de las Ciencias 3, Madrid, E-28040, Spain
3. Faculty of Computer Science, Complutense University, Profesor García Santesmases s/n, Madrid, E-28040, Spain
4. Department of Mathematics and Physics, Catholic University, Via Musei 41, Brescia, I-25121, Italy