1. Unidade Acadêmica de Matemática, Federal University of Campina Grande, 58429-900, Campina Grande, PB, Brazil
2. Department of Mathematics, Federal University of Paraíba, 58051-900 João Pessoa, PB, Brazil
3. Department of Mathematics, Federal University of Juiz de Fora, 36036-330 Juiz de Fora, Minas Gerais, Brazil