ABC formula and R-operation for quantum processes with unstable states

For a wide class of Hamiltonians used in quantum field theory and statistical physics, we obtain an explicit formula describing the behavior of the vacuum expectation of the evolution operator on large, but finite, time intervals. This formula also holds for processes with unstable states. __________ Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 149, No. 2, pp. 183–201, November, 2006.