  In this paper, we determine representatives for the mutation classes of
skew-symmetrizable 3x3 matrices and associated graphs using a natural
minimality condition, generalizing and strengthening results of
Beineke-Brustle-Hille and Felikson-Shapiro-Tumarkin. Furthermore, we obtain a
new numerical invariant for the mutation operation on skew-symmetrizable
matrices of arbitrary size.
