A variational method for quantum kinetic equations

A variational principle is formulated for the Kadanoff-Baym generalized qua ntum kinetic equations linearized with respect to external field, which des cribe electrons scattered on an arbitrary static potential. It is assumed t hat the Sigma (<) and <Sigma>(>) parts of the mass operator are arbitrary a nalytic functionals of the g(<) and g(>) correlation functions, and that th e character of analytic relations between <Sigma>(>), Sigma (<), g(>), and g(<) does not change in the presence of an external electric field. A close d system of integro-differential equations For correlation functions with k ernels determined by equilibrium functional dependences <delta>Sigma (alpha )/deltag(beta) (alpha, beta = <,>) is obtained. The existence of a quadrati c functional which attains minimum values on the exact solutions to this sy stem of equations is proved.