@article{10_4153_CJM_2016_050_1,
     author = {Hakl, Robert and Zamora, Manuel},
     title = {Periodic {Solutions} of an {Indefinite} {Singular} {Equation} {Arising} from the {Kepler} {Problemon} the {Sphere}},
     journal = {Canadian journal of mathematics},
     pages = {173--190},
     year = {2018},
     volume = {70},
     number = {1},
     doi = {10.4153/CJM-2016-050-1},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-2016-050-1/}
}