  This paper is concerned with the out-of-equilibrium two-lead Kondo model,
considered as a model of a quantum dot in the Kondo regime. We revisit the
perturbative expansion of the dot's magnetization, and conclude that, even at
order 0 in the Kondo interactions, the magnetization is not given by the usual
equilibrium result. We use the Schwinger-Keldysh method to derive a Dyson
equation describing the steady state induced by the voltage between the two
leads, and thus present the correct procedure for calculating perturbative
expansions of steady-state properties of the system.
