Destruction of invariant circles for Gevrey area-preserving twist maps
  In this paper, we show that for exact area-preserving twist maps on annulus, the invariant circles with a given rotation number can be destroyed by arbitrarily small Gevrey-$\alpha$ perturbations of the integrable generating function in the $C^r$ topology with $r<4-\frac{2}{\alpha}$, where $\alpha>1$. 
Lin Wang
math.DS