@article{pereira_2002,  title={Small amplitude oscillations of sine-gordon vortex states in planar magnets in three dimensions},  volume={32},  ISSN={0103-9733},  url={https://doi.org/10.1590/S0103-97332002000400025},  DOI={10.1590/S0103-97332002000400025},  number={3},  journal={Brazilian Journal of Physics},  publisher={Sociedade Brasileira de Física},  author={Pereira, A . R.},  year={2002},  month={Sep},  pages={815–820} }