  We study the non-equilibrium dynamics of a Tonks-Girardeau gas released from
a parabolic trap to a circle. We present the exact analytic solution of the
many body dynamics and prove that, for large times and in a properly defined
thermodynamic limit, the reduced density matrix of any finite subsystem
converges to a generalized Gibbs ensemble. The equilibration mechanism is
expected to be the same for all one-dimensional systems.
