@article{IM2_1991_36_3_a5,
     author = {A. T. Fomenko and H. Zieschang},
     title = {A topological invariant and a criterion for the equivalence of integrable {Hamiltonian} systems with two degrees of freedom},
     journal = {Izvestiya. Mathematics},
     pages = {567--596},
     year = {1991},
     volume = {36},
     number = {3},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1991_36_3_a5/}
}