Necessary and sufficient conditions for robust stabilization of an observer-based compensating system suffering nonlinear time-varying perturbation

It is demonstrated that the robust stabilization against additive nonlinear time-varying uncertainty presented by B.S. Chen and W.S. You (ibid., vol.AC-32, p.1131-5, Dec. 1987) is not just a sufficient but also a necessary condition. An additional necessary and sufficient condition that arises in the case of multiplicative uncertainty is also discussed.<
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