Faculty of Pharmacy, Department of Mathematics and Computer Science, University of Catania, Catania, Italy. francesco@dmi.unict.it|Faculty of Pharmacy, Department of Mathematics and Computer Science, University of Catania, Catania, Italy. francesco@dmi.unict.it|Faculty of Pharmacy, Department of Mathematics and Computer Science, University of Catania, Catania, Italy. francesco@dmi.unict.it