1. Department of Applied Mathematics, State University of Campinas (Unicamp), Rua Sérgio Buarque de Holanda, N. 651, CEP 13083-859, Campinas - SP, Brasil
2. Department of Applied Mathematics, São Paulo State University (Unesp), Rua Cristóvão Colombo, N. 2265, CEP 15054-000, São José do Rio Preto - SP, Brasil