2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/90539We study the evolution of an oscillator interacting via the most general bilinear coupling (with time-independent coefficients) with an ``environment'' consisting of a set of other harmonic oscillators. We are mainly interested in a possibility of using the Fokker-Planck equation to describe this evolution. Studying different interaction Hamiltonians, we show that unambiguous reduction to the Fokker-Planck equation is possible only within the framework of the so called rotating-wave approximation. As special cases we consider in detail the evolution of two coupled oscillators and relaxation of a charged oscillator in a uniform magnetic field.LaTex, 22 pages. This is the third part (pages 32-56) of the paper by the authors entitled "Quantum nonstationary oscillator: models and applications"Quantum Physicstext