@article{SM_1997_188_7_a6,
     author = {O. E. Orel},
     title = {A criterion for orbital equivalence of integrable {Hamiltonian} systems in the~vicinity of elliptic orbits. {An~orbital} invariant in {the~Lagrange} problem},
     journal = {Sbornik. Mathematics},
     pages = {1085--1105},
     publisher = {mathdoc},
     volume = {188},
     number = {7},
     year = {1997},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1997_188_7_a6/}
}