  A finite number of harmonic oscillators coupled to infinitely many
environment oscillators is fundamental to the problem of understanding quantum
dissipation of a small system immersed in a large environment. Exact operator
solution as a function of time is given to this problem, by using diagonalized
dynamical variable of the entire system, the small system plus the environment.
The decay law of prepared initial configuration is worked out in greatest
detail. A clear separation of the exponential- and the power-law decay period
is made possible by our method. Behavior of physical quantities at
asymptotically late times can be understood in terms of the overlap probability
of the system variable with the diagonal variable of the entire system.
