1. Faculty of Mathematics, Federal University of Uberlândia, Avenida João Naves de Ávila, 2.121, 38400-902 Uberlândia, MG, Brazil
2. Department of Mathematics, Instituto Nacional de Ciencias Agrícolas (INCA), km 3.5, San José de Las Lajas, 3700 Habana, Cuba
3. Department of Exact Sciences, University of São Paulo, Avenida Pádua Dias, 11, 13418-900 Piracicaba, SP, Brazil