@article{ELMA_1992__47_3_141533,
     author = {Norbert Hungerb\"uhler},
     title = {Geometrical {Aspects} of the {Circular} {Billiard} {Problem.}},
     journal = {Elemente der Mathematik},
     pages = {114--117},
     publisher = {mathdoc},
     volume = {47},
     number = {3},
     year = {1992},
     zbl = {0800.70052},
     url = {http://geodesic.mathdoc.fr/item/ELMA_1992__47_3_141533/}
}