On the nonequilibrium diagram technique: derivation, some features and applications
Lebedev Physical Institute, Russian Academy of Sciences, Leninsky prosp. 53, Moscow, 119991, Russian Federation
The logic of the nonequilibrium diagram technique (Keldysh diagram technique) appearance is reviewed. Simple examples are used to illustrate the implementation of the technique and to demonstrate possible difficulties and how to overcome them. Together with the well known facts, some lesser-discussed details are considered, in particular, whether the so-called three-component technique is necessary. A number of examples of nonequilibrium diagram technique (NDT) applications are given, including in particular tunneling systems, the linear response and other problems. We hope that some parts of the review will be useful even for those people for whom nonequilibrium diagram technique is well familiar.