@article{SM_1994_77_2_a15,
     author = {L. M. Lerman and Ya. L. Umanskii},
     title = {Classification of four-dimensional integrable {Hamiltonian} systems and {Poisson} actions of~$\mathbb{R}^2$ in extended neighborhoods of simple singular {points.~I}},
     journal = {Sbornik. Mathematics},
     pages = {511--542},
     publisher = {mathdoc},
     volume = {77},
     number = {2},
     year = {1994},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1994_77_2_a15/}
}