@article{novaes_2013,  title={Perturbed damped pendulum: finding periodic solutions via averaging method},  volume={35},  ISSN={1806-1117},  url={https://doi.org/10.1590/S1806-11172013000100014},  DOI={10.1590/S1806-11172013000100014},  number={1},  journal={Revista Brasileira de Ensino de Física},  publisher={Sociedade Brasileira de Física - SBF},  author={Novaes, Douglas D.},  year={2013},  month={Jan},  pages={01–07} }