  Multiplication of two elements of the special unitary group SU(N) determines
uniquely a third group element. A BAker-Campbell-Hausdorff relation is derived
which expresses the group parameters of the product (written as an exponential)
in terms of the parameters of the exponential factors. This requires the eigen-
values of three (N-by-N) matrices. Consequently, the relation can be stated
analytically up to N=4, in principle. Similarity transformations encoding the
time evolution of quantum mechanical observables, for example, can be worked
out by the same means.
