1. Department of Applied Mathematics, University of Campinas, Campinas-SP, Brazil.
2. Federal Institute of Goiás, Goiânia-GO, Brazil and Department of Applied Mathematics, University of São Paulo, São Paulo-SP, Brazil.
3. Institute of Mathematics and Statistics, Federal University of Goiás, Goiânia-GO, Brazil.
4. Department of Applied Mathematics, University of São Paulo, São Paulo-SP, Brazil.