  We discuss the formal relationship between the real-time Keldysh and
imaginary-time theory for nonequilibrium in quantum dot systems. The latter can
be reformulated using the recently proposed Matsubara voltage approach. We
establish general conditions for correct analytic continuation procedure on
physical observables, and apply the technique to the calculation of static
quantities in steady-state non-equilibrium for a quantum dot subject to a
finite bias voltage and external magnetic field. Limitations of the Matsubara
voltage approach are also pointed out.
