On the calculation of the linear stability parameter of periodic orbits
  In this paper we propose an improved method for calculating Henon's stability parameter, which is based on the differential of the Poincare map using the first variational equation. We show that this method is very accurate and give some examples where it gives correct results, while the previous method could not cope. 
C. Barbera (U. Rovira i Virgili-Spain & Obs. Marseille-France), E.   Athanassoula (Obs. Marseille-France)