We study the model of a strongly non-linear chain of particles coupled to two
heat baths at different temperatures. Our main result is the existence and
uniqueness of a stationary state at all temperatures. This result extends those
of Eckmann, Pillet, Rey-Bellet to potentials with essentially arbitrary growth
at infinity. This extension is possible by introducing a stronger version of
H\"ormander's theorem for Kolmogorov equations to vector fields with
polynomially bounded coefficients on unbounded domains.Comment: ~60 pages, 3 figure