1. Department of Mathematics and Informatics , University of Catania , Viale A. Doria 6, 95125 Catania , Italy
2. DISIM - Department of Information Engineering , Computer Science and Mathematics, University of L’Aquila , Via Vetoio snc - Coppito, 67100 , L’Aquila , Italy
3. Center for Research and Development in Mathematics and Applications, Department of Mathematics , University of Aveiro , 3810-193 , Aveiro , Portugal