A formalism is developed for finding the stationary states of systems for which radiative damping effects must be considered. The method includes a criterion for deciding whether or not a given unperturbed state has an exact stationary state corresponding to it. The method is applied to a simple example, the Lee model with unstable $V$ particle, and it is indicated how more complicated problems may be treated.

• Received 21 August 1958

DOI:https://doi.org/10.1103/PhysRev.112.1843