  We analyze the spectral function of the single-impurity two-terminal Anderson
model at finite voltage using the recently developed diagrammatic quantum Monte
Carlo technique as well as perturbation theory. In the
(particle-hole-)symmetric case we find an excellent agreement of the numerical
data with the perturbative results of second order up to interaction strengths
$U/\Gamma \approx 2$, where $\Gamma$ is the transparency of the
impurity-electrode interface. The analytical results are obtained in form of
the nonequilibrium self-energy for which we present explicit formulas in the
closed form at arbitrary bias voltage. We observe an increase of the spectral
density around zero energy brought about by the Kondo effect. Our analysis
suggests that a finite applied voltage $V$ acts as an effective temperature of
the system. We conclude that at voltages significantly larger than the
equilibrium Kondo temperature there is a complete suppression of the Kondo
effect and no resonance splitting can be observed. We confirm this scenario by
comparison of the numerical data with the perturbative results.
