Robust tracking: relation between the numerators, mode readability, and inverse internal model

In the robust, multivariable, asymptotic tracking problem with two-output plants, it is shown that if the numerator related to the controlled output is fixed, while the other numerator and the denominator are perturbed, then there is no solution to the problem. If, however, the whole part of the plant related to the controlled output is stable and fixed, while the other part is arbitrarily perturbed, the problem has a solution and the compensator that solves the problem incorporates an "inverse internal model" of the signal to be tracked.