@article{IM2_2017_81_4_a0,
     author = {S. V. Bolotin and V. V. Kozlov},
     title = {Topology, singularities and integrability in {Hamiltonian} systems with two degrees of freedom},
     journal = {Izvestiya. Mathematics },
     pages = {671--687},
     publisher = {mathdoc},
     volume = {81},
     number = {4},
     year = {2017},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_2017_81_4_a0/}
}