Comments on "A necessary and sufficient condition for stability of a perturbed system" by Q. Huang and R. Liu

It is pointed out that for a given controller, the sets of additive, multiplicative, and stable-factor perturbations that the plant can have without destabilizing the control system can be completely parametrized by using D.C. Youla's parametrization and by noting that the roles that the plant and the controller play in a standard feedback control configuration can be reversed.<
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