@article{IM2_1973_7_3_a4,
     author = {A. B. Katok},
     title = {Ergodic perturbations of degenerate integrable {Hamiltonian} systems},
     journal = {Izvestiya. Mathematics },
     pages = {535--571},
     publisher = {mathdoc},
     volume = {7},
     number = {3},
     year = {1973},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1973_7_3_a4/}
}