1. Department of Mathematics; University of Aveiro; 3810-193 Aveiro Portugal
2. LIA; University of Avignon; 84911 Avignon France
3. Department of Applied Mathematics; University of Campinas; 13081-970 Campinas Brazil
4. COPPE & Institute of Mathematics; Federal University of Rio de Janeiro; P.O. Box 68511 21941-972 Rio de Janeiro Brazil