On stability radii of infinite-dimensional time-varying discrete--time systems

This note introduces a stability radius for discrete–timelinear time–varying systems on Banach spaces under structuredtime–varying perturbations of multiple–output feedbacktype. Additive perturbations are considered which can be representedas a series of infinitely many perturbation terms. We derivecounterparts to some results established for time–varyingdifferentiable systems in [5, 7] and apply the results to periodicsystems.