Ahana Chakraborty, Rajdeep Sensarma, “Power-law tails and non-Markovian dynamics in open quantum systems: An exact solution from Keldysh field theory”, Phys. Rev. B, 97, № 10, 2018, 104306
J. SEKE, A. V. SOLDATOV, N. N. BOGOLUBOV, “APPLICATION OF THE SEKE SELF-CONSISTENT PROJECTION OPERATOR METHOD IN THE CASE OF A DISCRETIZED ATOM-FIELD INTERACTION MODEL”, Int. J. Mod. Phys. B, 13, № 26, 1999, 3159