  Based on the well known fact that the quasinormal frequencies are the poles
of the frequency domain Green's function we describe a method that allows us to
calculate exactly the quasinormal frequencies of the Klein-Gordon field moving
in the three-dimensional rotating BTZ black hole. These quasinormal frequencies
are already published and widely explored in several applications, but we use
this example to expound the proposed method of computation. We think that the
described procedure can be useful to calculate exactly the quasinormal
frequencies of classical fields propagating in other backgrounds. Furthermore,
we compare with previous results and discuss some related facts.
