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@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/} }
Jörg Waldvogel. The Problem of the Circular Billiard.. Elemente der Mathematik, Tome 47 (1992) no. 3, pp. 108-113. http://geodesic.mathdoc.fr/item/ELMA_1992__47_3_141532/