1. Department of Mathematics, Institute of Mathematics and Computer Science, University of São Paulo São Carlos, SP, Brasil
2. Av. Trabalhador São-Carlense, 400, Centro, Caixa Postal: 668, CEP: 13560-970, São Carlos, SP, Brazil
3. Department of Applied Mathematics, Institute of Technological and Exact Sciences, Federal University of Triângulo Mineiro, Av. Randolfo Borges Jnior, 1400, Univerdecidade, CEP: 38064-200, Uberaba, MG, Brazil