@incollection{AST_1983__98-99__74_0,
     author = {Braaksma, B. L. J. and Broer, H. W.},
     title = {Quasi periodic flow near a codimension one singularity of a divergence free vector field in dimension four},
     booktitle = {Bifurcation, th\'eorie ergodique et applications - 22-26 juin 1981},
     series = {Ast\'erisque},
     pages = {74--142},
     publisher = {Soci\'et\'e math\'ematique de France},
     number = {98-99},
     year = {1982},
     mrnumber = {724444},
     zbl = {0517.58033},
     language = {en},
     url = {http://archive.numdam.org/item/AST_1983__98-99__74_0/}
}