The general mixed mu problem has been shown to be NP hard, so that the exact solution of the general problem is computationally intractable, except for small problems. In this paper we consider not the general problem, but a particular special case of this problem, the rank one mixed mu problem. We show that for this case the mixed mu problem is equivalent to its upper bound (which is convex), and it can in fact be computed easily (and exactly). This special case is shown to be equivalent to the so-called ’affine parameter (for a polynomial with perturbed has been examined in detail in the literature, and for which several celebrated ’Kharitonov-type’ results have been proven.