  Prethermalization refers to the remarkable relaxation behavior which an
integrable many-body system in the presence of a weak integrability-breaking
perturbation may exhibit: After initial transients have died out, it stays for
a long time close to some non-thermal steady state, but on even much larger
time scales it ultimately switches over to the proper thermal equilibrium
behavior. By extending Deutsch's conceptual framework from Phys. Rev. A 43,
2046 (1991), we analytically predict that prethermalization is a typical
feature for a very general class of such weakly perturbed systems.
