@article{IM_1989__97_2_143703,
     author = {M.L., Polterovich, L.V. Bialy},
     title = {Lagrangian singularities of invariant tori of hamiltonian systems with two degrees of freedom.},
     journal = {Inventiones mathematicae},
     pages = {291--304},
     publisher = {mathdoc},
     volume = {97},
     number = {2},
     year = {1989},
     zbl = {0675.58016},
     url = {http://geodesic.mathdoc.fr/item/IM_1989__97_2_143703/}
}