On the propagation of a perturbation in an anharmonic system
  We give a not trivial upper bound on the velocity of disturbances in an infinitely extended anharmonic system at thermal equilibrium. The proof is achieved by combining a control on the non equilibrium dynamics with an explicit use of the state invariance with respect to the time evolution. 
P. Butt\`a, E. Caglioti, S. Di Ruzza and C. Marchioro