  The two-particle problem within a nonequilibrium many-particle system is
investigated in the framework of real-time Green's functions. Starting from the
dynamically screened ladder approximation of the nonequlibrium Bethe-Salpeter
equation, a nonequilibrium Dyson equation is given for two-time two-particle
Green's functions. Thereby the well-known Kadanoff-Baym equations are
generalized to the case of two-particle functions. The two-time structure of
the equations is achieved in an exact way avoiding the so-called Shindo
approximation. For the case of thermodynamic equilibrium, the differences to
former results obtained for the effective two-particle hamiltonian are
discussed.
