  An exact general formula for the matrix elements of the evolution operator in
quantum theory is established. The formul ("ABC-formula") has the form
<U(t)>=exp(At+B+C(t)). The constants A and B and the decreasing function C(t)
are computed in perturbation theory. The ABC-formula is valid for a general
class of Hamiltonians used in statistical physics and quantum field theory. The
formula includes the higher order corrections to the well known
Weisskopf-Wigner approximation and to the stochastic (van Hove) limit which are
widely used in considerations of problems of radiation, decay, quantum
decoherence, derivation of master and kinetic equations etc. The function C(t)
admits an interpretation as an analogue of the autocorrelation function
describing quantum chaos for the quantum baker's map.
