@article{MO_2014_69_1_a4,
     author = {F. D. Pukhovich},
     title = {External billiards},
     journal = {Matemati\v{c}eskoe obrazovanie},
     pages = {42--57},
     publisher = {mathdoc},
     volume = {69},
     number = {1},
     year = {2014},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MO_2014_69_1_a4/}
}