[1] Univ Fed Minas Gerais, Inst Ciencias Exatas, BR-30161970 Belo Horizonte, MG, Brazil
[2] Univ Fed Paraiba, Dept Matemat, BR-58059900 Joao Pessoa, Paraiba, Brazil
[3] Univ Fed Alagoas, Campus Sertao,Rodovia AL 145, BR-57480000 Delmiro Gouveia, Alagoas, Brazil
[4] Univ Fed Alagoas, Inst Matemat, Campus AC Simoes,BR 104-Norte,Km 97, BR-57072970 Maceio, Alagoas, Brazil