@article{IM_1992__108_2_144004,
     author = {N\'andor Sim\'anyi},
     title = {The {K-property} of {N} billiard balls {I.}},
     journal = {Inventiones mathematicae},
     pages = {521--548},
     publisher = {mathdoc},
     volume = {108},
     number = {2},
     year = {1992},
     zbl = {0759.58029},
     url = {http://geodesic.mathdoc.fr/item/IM_1992__108_2_144004/}
}