Department of Mathematics and Computer Science and Department of Clinical and Molecular Biomedicine, University of Catania, Catania, Italy.|Department of Mathematics and Computer Science and Department of Clinical and Molecular Biomedicine, University of Catania, Catania, Italy.|Department of Mathematics and Computer Science and Department of Clinical and Molecular Biomedicine, University of Catania, Catania, Italy.|Department of Mathematics and Computer Science and Department of Clinical and Molecular Biomedicine, University of Catania, Catania, Italy.