  Nonequilibrium quantum mechanics can be solved with the Keldysh formalism,
which evolves the quantum mechanical states forward in time in the presence of
a time-dependent field, and then evolves them backward in time, undoing the
effect of the time-dependent field. The Feynman path integral over the Keldysh
contour is employed to calculate the strongly correlated Green's function. We
examine the accuracy of this procedure for the simplest problem that requires a
nonequilibrium formulation: the f-electron spectral function of the spinless
Falicov-Kimball model.
