@article{RM_1991_46_5_a15,
     author = {Ya. B. Vorobets and G. A. Gal'perin and A. M. Stepin},
     title = {Periodic billiard trajectories in polygons},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     pages = {204--205},
     publisher = {mathdoc},
     volume = {46},
     number = {5},
     year = {1991},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/RM_1991_46_5_a15/}
}