Critical Attractor and Universality in a Renormalization Scheme for   Three Frequency Hamiltonian Systems
  We study an approximate renormalization-group transformation to analyze the breakup of invariant tori for three degrees of freedom Hamiltonian systems. The scheme is implemented for the spiral mean torus. We find numerically that the critical surface is the stable manifold of a critical nonperiodic attractor. We compute scaling exponents associated with this fixed set, and find that they can be expected to be universal. 
C. Chandre and H.R. Jauslin