  We study the nonequilibrium spectral function of the single-impurity Anderson
model connecting with multi-terminal leads. The full dependence on frequency
and bias voltage of the nonequilibrium self-energy and spectral function is
obtained analytically up to the second-order perturbation regarding the
interaction strength $U$. High and low bias voltage properties are analyzed for
a generic multi-terminal dot, showing a crossover from the Kondo resonance to
the Coulomb peaks with increasing bias voltage. For a dot where the
particle-hole symmetry is not present, we construct a current-preserving
evaluation of the nonequilibrium spectral function for arbitrary bias voltage.
It is shown that finite bias voltage does not split the Kondo resonance in this
order, and no specific structure due to multiple leads emerges. Overall bias
dependence is quite similar to finite temperature effect for a dot with or
without the particle-hole symmetry.
