Geodesic
Asymptotics of means in a non-equilibrium model of the Dicke type
Zapiski Nauchnykh Seminarov POMI, Questions of quantum field theory and statistical physics. Part 7, Tome 161 (1987), pp. 139-145 
S. A. Fedotov. Asymptotics of means in a non-equilibrium model of the Dicke type. Zapiski Nauchnykh Seminarov POMI, Questions of quantum field theory and statistical physics. Part 7, Tome 161 (1987), pp. 139-145. http://geodesic.mathdoc.fr/item/ZNSL_1987_161_a10/
@article{ZNSL_1987_161_a10,
     author = {S. A. Fedotov},
     title = {Asymptotics of means in a non-equilibrium model of the {Dicke} type},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {139--145},
     year = {1987},
     volume = {161},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1987_161_a10/}
}
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The representation of the density matrix f o r the nonequilibrium system in terms of functional integrals is used for obtaining and the rigorous proof the asymptotic formulae for the averages of the radiation field interacting with a system of two-level atoms at $N\to\infty$ ($N$ is a number of atoms). The estimation method for functional integrals developed earlier for the equilibrium model system is used in the rigorous proof of the asymptotic formulae.