@article{GDZPPN002082985, title = {The Problem of the Circular Billiard.}, journal = {Elemente der Mathematik}, volume = {47}, year = {1992}, pages = {108-113}, zbl = {0800.70051}, url = {http://dml.mathdoc.fr/item/GDZPPN002082985} }
Waldvogel, Jörg. The Problem of the Circular Billiard.. Elemente der Mathematik, Tome 47 (1992) pp. 108-113. http://gdmltest.u-ga.fr/item/GDZPPN002082985/