Bounded Real Lemma for Structured Noncommutative Multidimensional Linear Systems and Robust Control

We obtain a standard Bounded Real Lemma and a strict Bounded Real Lemma for the class of input/state/output linear systems with evolution along a free semigroup recently introduced by the authors. As an application we show the equivalence of this result with a recent result from robust control theory: the structured μ-singular value of an operator (taken in a certain uniform sense) is equal to its upper bound computed via diagonal scalings in case the structured singular value is taken with respect to the collection of time-varying perturbations given by a linear-fractional model.