@article{article7cc3d11c,
title   = {On the existence of periodic solutions for the equation x 'DOIS PONTOS'+f (x) x 'PONTO'+g (x) =0},
author = {Guidorizzi, Hamilton L.},
year = {1991},
doi = {10.1007/BF01244899},
journal   = {Boletim da Sociedade Brasileira de Matemáica}
}