1. Dipartimento P.A.U., Università Degli Studi Mediterranea di Reggio Calabria, Salita Melissari - Feo di Vito, 89100 Reggio Calabria, Italy
2. Institute of Mathematics "Simion Stoilow" of the Romanian Academy, 014700 Bucharest, Romania
3. Department of Mathematics, University of Craiova, 200585 Craiova, Romania