@article{ELMA_1992__47_3_141532,
     author = {J\"org Waldvogel},
     title = {The {Problem} of the {Circular} {Billiard.}},
     journal = {Elemente der Mathematik},
     pages = {108--113},
     publisher = {mathdoc},
     volume = {47},
     number = {3},
     year = {1992},
     zbl = {0800.70051},
     url = {http://geodesic.mathdoc.fr/item/ELMA_1992__47_3_141532/}
}