@article{JGG_2021_25_1_JGG_2021_25_1_a8,
     author = {H. Stachel },
     title = {Isometric {Billiards} in {Ellipses} and {Focal} {Billiards} in {Ellipsoids}},
     journal = {Journal for geometry and graphics},
     pages = {97--118},
     publisher = {mathdoc},
     volume = {25},
     number = {1},
     year = {2021},
     url = {http://geodesic.mathdoc.fr/item/JGG_2021_25_1_JGG_2021_25_1_a8/}
}