Limit cycle bifurcations from a nilpotent focus or center of planar   systems
  In this paper, we study the analytical property of the Poincare return map and the generalized focal values of an analytical planar system with a nilpotent focus or center. Then we use the focal values and the map to study the number of limit cycles of this kind of systems with parameters, and obtain some new results on the lower and upper bounds of the maximal number of limit cycles near the nilpotent focus or center. 
Maoan Han, Valery G. Romanovski
math.CA