Department of Mathematics and Computer Science - University of Catania, Catania, Italy.|Department of Mathematics and Computer Science - University of Catania, Catania, Italy.|Department of Mathematics and Computer Science - University of Catania, Catania, Italy.|Department of Mathematics and Computer Science - University of Catania, Catania, Italy.|Department of Mathematics and Computer Science - University of Catania, Catania, Italy.|Department of Mathematics and Computer Science - University of Catania, Catania, Italy.|Department of Mathematics and Computer Science - University of Catania, Catania, Italy.