@inproceedings{matthias2011closed, title={Closed magnetic geodesics on $S^2$}, author={Matthias Schneider}, url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20160912071122598399870}, booktitle={Journal of Differential Geometry}, volume={87}, number={2}, pages={343-388}, year={2011}, }