Boundary conditions for statistical operators in the theory of nonequilibrium processes and quasiaverages

It is shown that the boundary conditions for nonequllibrlum statistical operators can be formulated by introducing infinitesimally small sources which destroy the symmetry with respect to time reflection into the Liouville equation for the statistical operator (or its logarithm). These boundary conditions have a very close analogy with the boundary conditions which single out the retarded solutions of the Schrödinger equation in the quantum theory of scattering.