@article{SM_2012_203_8_a2,
     author = {O. A. Zagryadskii and E. A. Kudryavtseva and D. A. Fedoseev},
     title = {A generalization of {Bertrand's} theorem to surfaces of revolution},
     journal = {Sbornik. Mathematics},
     pages = {1112--1150},
     publisher = {mathdoc},
     volume = {203},
     number = {8},
     year = {2012},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_2012_203_8_a2/}
}