Let dot(x)=f(x,u) be a system of differential equations in Rn. When u=u0, assume that there is an equilibrium p for which the following hypotheses are satisfied:
a1: Dxf(p) has a simple eigenvalue o with right eigenvector v and left eigenvector w.
a3: w(Dxxf(p)(v,v)) 不等于零.