  (a) Let X=(f,g) be a differentiable map in the plane (not necessarily C^1)
and let Spec(X) be the set of (complex) eigenvalues of the derivative DX(p)
when p varies in R^2. If, for some \epsilon>0, the set Spec(X) is disjoint of
[0,\epsilon) then X is injective.
  (b) Let X be a differentiable vector field such that X(0)=0 and $Re(z)< 0$
for all z in Spec(X). Then, for all p in R^2, there is a unique positive
trajectory starting at p; moreover the \omega-limit set of p is equal to {0}.
