This paper describes perturbative framework, on the basis of closed-time-pathformalism, for studying quasiuniform relativistic quantum field systems nearequilibrium and nonequilibrium quasistationary systems. At the first part,starting from first principles, we construct perturbative schemes forrelativistic complex-scalar-field theory. We clarify what assumption isinvolved in arriving at a standard perturbative framework and to what extentthe $n (\geq 4)$-point initial correlation functions that are usually discardedin the standard framework can in fact be discarded. Two calculational schemesare introduced, the one is formulated on the basis of the initial particledistribution function and the one is formulated on the basis of the ``physical'' particle distribution function. Both schemes are equivalent and leadto a generalized relativistic kinetic or Boltzmann equation. At the secondpart, using the perturbative loop-expansion scheme for an $O (N)$ linear$\sigma$ model, we analyze how the chiral phase transition proceeds throughdisoriented chiral condensates. The system of coupled equations that governsthe spacetime evolution of the condensate or order-parameter fields is derived.The region where the curvature of the ``potential'' is negative is dealt withby introducing the random-force fields. Application to simple situations ismade.
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