@book{45019,
	author = {Roussarie, F., Dumortier and Roussarie, R.},
	title = {Bifurcations of planar vector fields: nilpotent singularities and abelian integrals},
	publisher = {Springer-Verlag},
	year = {1991},
	series = {Lecture notes in mathematics},
	address = {Berlin}
}