@article{SM_1992_71_1_a0,
     author = {I. K. Babenko},
     title = {Periodic trajectories in three-dimensional {Birkhoffs} billiards},
     journal = {Sbornik. Mathematics},
     pages = {1--13},
     publisher = {mathdoc},
     volume = {71},
     number = {1},
     year = {1992},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1992_71_1_a0/}
}