  We study the dynamical properties of the quantum Rabi model within a
systematic expansion method. Based on the observation that the parity symmetry
of the Rabi model is kept during the evolution of the states, we decompose the
initial state and the time-dependent one into a part of a positive and a
negative parity expanded by the superposition of the coherent states. The
evolutions for the corresponding positive and the negative parity are obtained,
where the expansion coefficients in the dynamical equations are known from the
recurrence relation derived.
