Optimal performance with plant uncertainty

This paper deals with the design of feedback controllers which minimize the H?? norm of the sensitivity matrix over an operating band, with the constraint that the feedback system remain stable under a class of multiplicative plant output perturbations. The matrix version of Nehari´s result concerning the distance of an L?? function to the Hardy space H?? is used to characterize the existence of H?? functions constrained over disjoint frequency bands.