KAM--renormalization-group for Hamiltonian systems with two degrees of   freedom
  We review a formulation of a renormalization-group scheme for Hamiltonian systems with two degrees of freedom. We discuss the renormalization flow on the basis of the continued fraction expansion of the frequency. The goal of this approach is to understand universal scaling behavior of critical invariant tori. 
C. Chandre, H.R. Jauslin