@article{MZ_1991__206_1_174233,
     author = {Hidekazu Ito},
     title = {Action-angle coordinates at singularities for analytic integrable systems.},
     journal = {Mathematische Zeitschrift},
     pages = {363--408},
     year = {1991},
     volume = {206},
     number = {1},
     zbl = {0707.58026},
     url = {http://geodesic.mathdoc.fr/item/MZ_1991__206_1_174233/}
}