Dumortier, F., et al <br/><br/>
Bifurcations of planar vector fields  nilpotent singularities & abelian integrals 
F. Dumortier, et al 
 - Berlin  Springer  1991 
 - viii  226 
 - Lecture notes in mathematics 1480 .
<br/><br/><label>ISBN: </label>3540545212 
<br/><br/><label>Dewey Class. No.: </label>6.07  / DUMO