Department of Mathematics, University of Aveiro, 3810-193 Aveiro, Portugal. vera@ua.pt|Department of Mathematics, University of Aveiro, 3810-193 Aveiro, Portugal. vera@ua.pt|Department of Mathematics, University of Aveiro, 3810-193 Aveiro, Portugal. vera@ua.pt|Department of Mathematics, University of Aveiro, 3810-193 Aveiro, Portugal. vera@ua.pt|Department of Mathematics, University of Aveiro, 3810-193 Aveiro, Portugal. vera@ua.pt