@article{AA_1989_1_3_a3,
     author = {S. V. Buyalo},
     title = {Closed geodesics on 2-dimensional orbifolds of nonpositive curvature},
     journal = {Algebra i analiz},
     pages = {89--109},
     publisher = {mathdoc},
     volume = {1},
     number = {3},
     year = {1989},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AA_1989_1_3_a3/}
}